Does anyone care to ask an amplifier designer a technical question? My door is open.


I closed the cable and fuse thread because the trolls were making a mess of things. I hope they dont find me here.

I design Tube and Solid State power amps and preamps for Music Reference. I have a degree in Electrical Engineering, have trained my ears keenly to hear frequency response differences, distortion and pretty good at guessing SPL. Ive spent 40 years doing that as a tech, store owner, and designer.
.
Perhaps someone would like to ask a question about how one designs a successfull amplifier? What determines damping factor and what damping factor does besides damping the woofer. There is an entirely different, I feel better way to look at damping and call it Regulation , which is 1/damping.

I like to tell true stories of my experience with others in this industry.

I have started a school which you can visit at http://berkeleyhifischool.com/ There you can see some of my presentations.

On YouTube go to the Music Reference channel to see how to design and build your own tube linestage. The series has over 200,000 views. You have to hit the video tab to see all.

I am not here to advertise for MR. Soon I will be making and posting more videos on YouTube. I don’t make any money off the videos, I just want to share knowledge and I hope others will share knowledge. Asking a good question is actually a display of your knowledge because you know enough to formulate a decent question.

Starting in January I plan to make these videos and post them on the HiFi school site and hosted on a new YouTube channel belonging to the school.


Fe8c8cab 4117 4c51 b1aa 6b134ad0dca6ramtubes
I will be the first. I never owned an amp of yours. I have owned other tube amps by several other manufacturers. I gave up on them, and this has been through a pair of modified and tweek'd pair of Klipsch Lascala. Why did I give up on them ? Noise, microphoncs, pretty often biasing, warm up time ( I leave ss amps continuously because of the superior sq ). and " timing " in the bass ( muddy ). I go passive through a Dac. Should I try an amp of yours, and why ? I listen loudly to rock and jazz. Thank you... Enjoy ! MrD.
BTW, I am believer in both pcs and fuses. Enjoy ! MrD.
Hello Mr Ramtubes

I wish to get your opinion about SET and Push Pull tube amplifiers.

Sometimes double SET using two parallel SET also come up..

I like the mid range roundness of SET tube amplifier but they have limitation in power.

Thomas


I have no particular question to ask at this time, Roger, but I want to take the opportunity to express my appreciation for your participation here. As with Ralph (Atmasphere) and a few other audio designers who participate here at least occasionally, I always follow your posts with interest, and I have learned from them. And I’ve learned from them even though I have BSEE and MSEE degrees myself, from excellent schools, as well as 33 years experience designing and managing design of sophisticated analog and digital circuits (not for audio), and nearly 40 years of experience as an audiophile.

Thank you, and best regards,
-- Al
Even though I disagree with you on the efficacy of some things, like power cords and fuses, welcome. It's too bad you had to shut down the other thread but it was getting out of hand.

All the best,
Nonoise
@mrdecibel

I will be the first. I never owned an amp of yours. I have owned other tube amps by several other manufacturers. I gave up on them, and this has been through a pair of modified and tweek’d pair of Klipsch Lascala. Why did I give up on them ? Noise, microphoncs, pretty often biasing, warm up time ( I leave ss amps continuously because of the superior sq ). and " timing " in the bass ( muddy ). I go passive through a Dac. Should I try an amp of yours, and why ? I listen loudly to rock and jazz. Thank you... Enjoy ! MrD.


First I’m not using this post to sell amplifiers, that is up to you. However I can understand and appreciate your frustration with the tube amps you have owned. Here is why.

Noise is simply due to designer who doesn’t hear or mind the noise, probably has no internal limits for what is low noise, or doesn’t know how to measure noise. It could be a noisy tube.

Microphonics are generally created in the first tube of the amp, the input tube. You just have to find a good one. I do grade tubes for noise and microphonics.

A good amplifier needs to have its bias adjusted perhaps twice a year or when you hear something change radically.

Good amplifiers warm up and are 99% there in about 5-10 minutes. There is no good reason to warm them up or leave them on. They play in the first 30 seconds and don’t sound bad as they come up slowly.

Muddy bass is generally due to low damping factor. Get a damping factor of at least 8-10. More than that wont make much difference and high damping tube amps are often unstable due to too much feedback.

BTW, I am believer in both pcs and fuses. Enjoy ! MrD
Thats fine with me. Just don't use premium fuses in the B+ circuit of any power amp. There is a lot of energy stored in the filter caps and it has to go somewhere. A high interrupting fuse does the proper job of leaving most of the energy in the filter caps rather than tearing a path of destruction through your amplifier..
Roger, normally things go pretty easy on this forum unless certain individuals get involved as you've already sorted out. When they jump in, that's usually a signal that the usefulness of the thread is over.
An extremely generous offer. Thank you.

What is it, from a technical standpoint, that allows good tube amplification to reproduce micro dynamics in a way that is much closer to what I hear in live music? I refer to the ability to reproduce the seamless dynamic gradations in volume (especially very subtle gradations) that give music a sense of aliveness; not simply the ability to play very loudly. I know some will disagree, but to me tube amps generally do a better job of this than solid state. The attributes of tube amplification are often described in terms of tonality, dimensionality, staging, but the thing that draws me to tubes more than anything is the way they reproduce dynamics. I generally hear more dynamic nuance (realism) from tube amplification. Thoughts?
Post removed 
I have been checking out some of your videos on YouTube, especially the one at burning amp I think it was. Loved it!  Really appreciate all your doing. 
@frogman An extremely generous offer. Thank you.
What is it, from a technical standpoint, that allows good tube amplification to reproduce micro dynamics in a way that is much closer to what I hear in live music? I refer to the ability to reproduce the seamless dynamic gradations in volume (especially very subtle gradations) that give music a sense of aliveness; not simply the ability to play very loudly. I know some will disagree, but to me tube amps generally do a better job of this than solid state. The attributes of tube amplification are often described in terms of tonality, dimensionality, staging, but to me the thing that draws me to tubes more than anything is the way they reproduce dynamics. I generally hear more dynamic nuance (realism) from tubes. Thoughts?


Transistors have a very abrupt turn on. A silicon transistor starts conducting with 0.6 volts on the input (base) and is fully on by 0.7. With MOSFETS the numbers are just as bad 3.1 to 3.5 or so.

Tubes, on the otherhand, have in input range of many volts. A preamp tube might go from -10 to 0 on the input. A power tube -50 to 0. The - sign occurrs because tubes want to be on till you cut them off, so the grid (input) always starts out negative.

So input range has something to do with producing microdynamics and dynamic range as the devices themselves inherently have more range.

One other thing that has drawn me to tubes. Only with tubes can you make a minimal circuit that sound good. Transistors require many stages and lots of feedback. Now feedback is not inherently bad but a lot of feedback gets people in trouble with oscillations (instability). I also like the fact that tubes need no protective circuitry, last long if well treated, are nice to look at.

Transistors don’t last forever and transistor amps are much harder to fix than most tube amps. (not ARC) :(

In the new year, time permitting, I will put up some videos to show people how easy it is to fix a tube amp. The other day I fixed one over the phone in Germany. With proper knowledge and a little advice its not hard.
I have a question for you Roger:
What are your thoughts on Class D amps? I have listened to quite a few and always came away unimpressed with my ears aching. I am a Mac guy at heart marrying a Mac solid state amp with a Mac tube preamp. Perhaps I like a more mellower sound?

I heard your tube amps back in the 90’s at Sound II in North Dartmouth, Ma. Leo (RIP) was a huge fan of tubes and carried your line as well as VAC and Audible Illusions. I did a lot of listening and buying in that store.  An added plus was there was a fantastic strip Joint right across the street from the store on Route 6.  Really fun times!
Roger, thank you for the response. I have to disagree with you as to warm up time of a component ( which is relative to break in time ). For me, 5-10 minutes is never enough. " Don't sound bad when they come up slowly ". For me it takes much more time. I am not talking measurements, I am talking " listening ". I would like to leave it at that, as this, too, could wind up being a very controversial discussion, and not new to Audiogon. Enjoy ! MrD.
@djones

 I have never owned a tube amp , always used SS and this doesn't come up if it's rated for 8 and 4 Ohm. I have what is I am sure a novice question. My speakers are 6 Ohm and I notice most tube amps have 4 Ohm or 8 Ohm taps and sometimes 2 Ohm. Where would I connect a 6 Ohm speaker?


Here is where experimentation will produce very real and obvious results. Always use the lowest tap that gives you enough volume without distortion (clipping). You will then get the most damping and the speaker will have frequency response as close as possible to the designers voicing. If you want more detail on why that is ask me. Its long.

Also experiment with the taps at low listening levels where clipping will not be an issue. Listen for tonal changes and peaky or mushy bass. The bass is most affected. 

FInd an impedance curve for your speaker and send me a link. Impedance variations are the largest cause of response variations with most tube amps and tap choices.
Welcome Roger and thank you.

Just a point in case this also gets trashed by you know who,s.

The relatively new Insiders Lobby allows you to post a thread and I believe block certain members from adding dross to it.

just a thought n case the worst happens.
Now a novice question.

i have zero tube amp experience.

a lot of my equipment runs xlr connections 

i have not seen many tube integrated amps sporting xlr inputs.

is there a good reason for this?
Roger, great advice!  When I owned a pair of Harbeth C7s, I ran into the same issue. The speakers were rated 6ohms. I was using a Rogue Cronus Magnum 2 and tested with both the 4ohms and 8ohms taps before deciding on 8ohms.  The bass was just better.  
Sorry I deleted my question. I thought it might be to much of a novice question. I knew 6 Ohms would be harder than 8Ohms but easier than 4 Ohm which is why I thought a 6 Ohm speaker might be hard on the amp connected to the 8 Ohm tap and might blow a speaker connected to the 4 Ohm tap. Thanks for the answer.
Hi Roger, thanks for the great offer. I would like to know what are all the measurable variables does one consider when manufacturing an amplifier.

I am guessing this list is most exhaustive then the spec sheet typically posted?. If not correct me.

For most of the lesser informed people buyers like me we only have spec sheet and most amps almost are always good with stated specs. So it is really hard to differentiate and choose.

Roger; How do you feel about the design characteristics of hybrid amps, say with a tube preamp and a solid state power amp like the Rogue Pharaoh?  Does this approach offer advantages of both tube and SS, or does it compromise the best of both sides?  Thanks in advance.
I will take you up on your offer.
A bit of background.
Having an interest in Loudspeaker Design / Acoustics / Physics.
HiFi is more than a hobby for me.
I'm old enough to have replaced / checked TV Set Tubes.
And young enough to be blown away each time I settle down to listen.
Being more familiar with DC circuits.
By comparison AC is Craazzee fun.
My Goal is to Design my Personal Loudspeaker System.
As my current interest began around 20 years ago.
My Design Ideas are a bit more comprehensive.
Leading to the Goal of an Active System.
Likely an All Active System.
Solid State likely as well.
The Electronics could be outboard.
Thinking 'Sound Reproduction' more than Creating a Sonic Palate.
Where do I begin when considering an Amplifiers Characteristics?

This is what makes a hobby in audio fun real people helping other people with knowledge and experience. Thank you kind sir. 
@mrdecibel 

Roger, thank you for the response. I have to disagree with you as to warm up time of a component ( which is relative to break in time ). For me, 5-10 minutes is never enough. " Don't sound bad when they come up slowly ". For me it takes much more time. I am not talking measurements, I am talking " listening ". I would like to leave it at that, as this, too, could wind up being a very controversial discussion, and not new to Audiogon. Enjoy ! MrD.


Was that MY amp that took hours? Not all tube amps are the same. However you are welcome to warm up as long as you like. There is no controversy. 

To help solve the warm up problem I put a forming switch on the RM-200 MK II at the suggestion of Richard Vandersteen. This switch keeps the caps formed beyond operating voltage. We both feel that cap forming is the most obvious reason for long warmups.  

One might also consider warm up time on their ears and other senses. When we first sit down we are not in the same state as some time later when we have relaxed and shut out the noise of the outside world.




Thank you Roger. You are a true asset to the audiophile community. Very generous.  
Thank you for your kind offer, Roger.

I build solid state amplifiers, and like to match output transistors. At first I matched for HFE, then VBE at constant current similar to operating spec. Only later did I realize how much this parameter drifts over the first hour of warmup.

However, my latest amps are Class A, and I suspect that a more realistic match is obtained by culling outliers by HFE, then match from VBE using the bias at constant potential and sufficient to generate the operating current. Finally, instead of using matched emitter resistors, I use emitter resistors tailored to the output devices, so that each emitter resistor sees the same potential drop.

Your thoughts? Any advice appreciated.
I wonder, how many have actually heard a Music Reference amp? Not a "trendy" brand, conservatively and timelessly styled (I happen to find the RM-200 minimalistily elegant, in a Scandinavian sort of way), few dealers, fewer reviews. I have found MR owners to be independent-minded, and secure in their own opinions, needing no validation from others. My kinda people.
@uberwaltz 

 Now a novice question.

i have zero tube amp experience.

a lot of my equipment runs xlr connections

i have not seen many tube integrated amps sporting xlr inputs.

is there a good reason for this?

Balanced inputs take a lot of extra stuff. Most integrateds are going to turn the balanced into unbalanced right off because of volume control issues. The extra stuff in SS amps is a 50 cent op amp. In tube amps it would another tube or transformer. Much more expensive.

Balanced makes more sense on power amps where it is actually easy to do because of the absence of a volume control. 

Keep in mind just because a unit has a XLR input that is no assurance that the input is balanced.


Roger, I assume the balanced/XLR-only input on your RM-200 is accomplished with a transformer. Do you wire it in accordance with AES File 48 (pin 1: ground, pin 2: non-inverted signal, pin 3: inverted signal)? Thanks---Eric. 
@rego Where do I begin when considering an Amplifiers Characteristics?

The first thing people are hearing, whether they believe it or not, is the difference in amplifier output impedance that will affect some speakers more than others. An amplifier with an output impedance greater than 1/10 the tap impedance will start to modify the frequency response of a speaker. The frequency response modification will simply follow the impedance curve of the speaker. 

There are some tube amps out there, that people love, that have an output impedance greater than the tap impedance and those will make very obvious changes in frequency response. The bass will always have a peak, the mids may have a dip, the highs may be accentuated or reduced, all dependent on the impedance curve. 

I purposely did not use the term "damping" because that term implies controlling the woofer which is actually not what is going on. However 1/10 the tap impedance or 1/10 the speaker impedance is a damping factor of 10.  I firmly believe that we should retire the term damping factor or get a better understanding of what is really going on. One has to remember that an 8 ohms speaker has typically 6 ohms of DC wire resistance in the voice coil. This makes the output impedance rather unimportant.

I expect some flack on this. So just be kind and logical and I will blow your mind about damping.


@bdp24 

Roger, I assume the balanced/XLR-only input on your RM-200 is accomplished with a transformer. Do you wire it in accordance with AES File 48 (pin 1: ground, pin 2: non-inverted signal, pin 3: inverted signal)? Thanks---Eric.


Actually it does not though I do use th AES standard for the pins. 

The balanced input is a differential pair with feedback right to the input so that the characteristics of the amplfier are determined largely by 4 high quality resistors.. and of course the rest of tube magic.

The CMRR is very high at 90 db and the amps works and sounds the same if driven unbalanced (single ended)
@terry9  I build solid state amplifiers, and like to match output transistors. At first I matched for HFE, then VBE at constant current similar to operating spec. Only later did I realize how much this parameter drifts over the first hour of warmup.

However, my latest amps are Class A, and I suspect that a more realistic match is obtained by culling outliers by HFE, then match from VBE using the bias at constant potential and sufficient to generate the operating current. Finally, instead of using matched emitter resistors, I use emitter resistors tailored to the output devices, so that each emitter resistor sees the same potential drop.
Your thoughts? Any advice appreciated.


Kudos to you for building your own gear. I have often said that the best amplifier is the one someone builds for himself. It brings along with it the joy that it is your creation not someone elses.

Of course you match HFE at operating current?

What is the spread of emitter resistor values? That part concerns me a little.

How many transistors in parallel?

Yes transistors are very temperature sensitive. VBE varies at -2 mV per degree C. Thus a 40 degree rise will be 80 MV and that is a lot to deal with. Do you have a VBE multiplier in the bias circuit?
Mr Ramtubes, I'm looking for some information about a stereo receiver I found in my late dad's basement. It's a "Thomas America's sound of Music"  I can't find anything about it on the web. When I came across it, it was buried behind a bunch of stuff with other stereo equipment pilled on top of it. I was able to lean in close enough to get a couple of pics with my phone so, unfortunately I don't have any other info (i.e. model#, serial #) at this time. Any info would be much appreciated. 
@rocknss
 Curious on how you have trained your ears.

Time, attention to distortion, frequency response variations, lots of A/B testing... lots of that. I set up one today to use a QUAD 57 to compare a stock with a modified amplifier that has come to interest me and may become a product.

One experience I would share is a listening session with 5 rather proud golden eared audiophiles and myself. They were all lined up on the couch in pretty good position to hear the system. I was 90 degrees off axis having some wine and cheese. I heard a horrible rendition of a stand up bass.

I first said, thats the worst bass fiddle I have ever heard, is this some horrible recording, no its a well respected recording. Then I asked, "what do your guys hear" They said audiophile things like, poor imaging, no depth, yada yada yada. I said, What i hear is a lot of distortion, like 30%.  The source was a tube modified OPPO player. I said, hey got some more 6SN7s around? So new tubes made everything fine. The host, a good friend, quickly brought up his tube tester and we found the old tubes were down to 20% emission.

The point of this story is that audiophiles rarely hear distortion as distortion. That word is not popular. They were listening to and judging a system that was simply broken. So the first thing people need to know is when their system is broken. I think many systems are.

In closing I think time, confidence (which is hard to get depending on who you hang with), and the Harman link are all good things. I know that Harman has developed some very good listening procedures and selects their listeners carefully. The qualification process to become one of their listeners is long and many people are eliminated. What Harman wants to get out of all this is to have listeners tell them if they are going in the right direction with changes in speakers and other components. Of course they use measurements also. Any sane person would.
I was measuring HFE with a handheld, not warm, not at operating current, and only using the result as a first cut to cull outliers.

Emitter resistors are 0R1 +/- 0R02, so not much.

4 x PNP and 4 x NPN.

No VBE multiplier, but then I leave my amps on most of the time, and they stabilize below 50C.

Thanks very much for taking an interest, Roger.

Thanks for the balanced info Roger. 4 resistors > transformer! I've been running the RM-200 with a balanced pre-amp, but if connected to an unbalanced source (First Watt B4 x/o), what is your recommended method of converting the RM-200's XLR jack for use with an RCA plug, or put another way, connecting an RCA plug to the XLR jack? I've seen a cable with XLR's at one end, RCA's at the other. Acceptable?
@geek101    Hi Roger, thanks for the great offer. I would like to know what are all the measurable variables does one consider when manufacturing an amplifier.

I am guessing this list is most exhaustive then the spec sheet typically posted?. If not correct me.

For most of the lesser informed people buyers like me we only have spec sheet and most amps almost are always good with stated specs. So it is really hard to differentiate and choose.


Good Question. One thing to do is follow John Atkinson's measurements of amplifiers. If you dont get Stereophile its a good mag and only $12-15 a year. If I get 10 new subscribers I get a toaster... :).. Just kidding

I find most amplifier specs to be missing something that should be there. Perhaps because that particular spec is not so good. So here is the list.

Distortion from 0.25 watts to full power over 20-20,000 cycles. This spec came from the FTC.

Damping factor can be most important thing if it is below 8-10. However much more than 10 doesnt make much difference. In other words, some SS amps have damping over 100. Thats ok, but at what sonic cost.  Damping factor will have a great effect on frequency response if your speaker has a widely varying impedance curve. Dont be impressed by super high damping. Its not going to matter.

-3dB power bandwidth. Jadis did not publish this because the spec would be horrible. Heres a 4 chassis amp that can do 200 watts but not to 20Khz. The spec 20-20Khz is more like 5 watts. I have measured it.

Noise often specified in db below full output. Not a good way to spec this. I would prefer a simple statement in noise in mV both wideband and A weighted. Good luck on that one. 

Power consumption will give you some idea of how much this thing is going to heat your room. 

That pretty much covers it. MTBF (mean time between failures) is speced on all parts of your computer, industrial power supplies and many things. Unfortunately I have never seen this spec on an amplifier. The number for most amps would be very low. MTBF for things like hard drives is 300,000 to 1,200,000 hours

 http://knowledge.seagate.com/articles/en_US/FAQ/174791en

The MTBF of amplifiers varies widely and is only known by those of us who fix them. I am very proud of the MTBF for my amplifiers. I can only state it in years and its at least 30 years. 

If you want to buy an amplifier that will not trouble you, use the internet and find what people have experienced.


Roger, thank you again for your response. My ears have required long warm up times for almost all of the gear I have had over the years. Solid State as well as tubes, dacs, tuners, tape machines, etc. I suppose R. V. knew what I was talking about, the warm up problems. Is the forming switch the same as an amp having a stand by mode ? I assume it is this capacitor thing that I have been responding to. I am glad you are here posting, as I am greatly enjoying the information. Enjoy ! MrD.
@terry9 

I was measuring HFE with a handheld, not warm, not at operating current, and only using the result as a first cut to cull outliers.

Emitter resistors are 0R1 +/- 0R02, so not much.

4 x PNP and 4 x NPN.

No VBE multiplier, but then I leave my amps on most of the time, and they stabilize below 50C.

50 C/122F is nicely cool, only 25 C above ambient in a standard room. You either have big heat sinks or not so much idle current.

Do you mean resistors from 0.1 ohms to 0.02 ohms? I like the American system of resistors.  That is a large range. Do your account for this and make the idle currents equal in each transistor?

HFE for a power transistor at low current is rather meaningless. Why not do all of it at operating current?

A VBE multiplier is a very good thing and easy to add. How much does your total idle current vary from cold to hot and how long does it take to get there? What is the idle dissipation?
@bdp24 

Thanks for the balanced info Roger. 4 resistors > transformer! I've been running the RM-200 with a balanced pre-amp, but if connected to an unbalanced source (First Watt B4 x/o), what is your recommended method of converting the RM-200's XLR jack for use with an RCA plug, or put another way, connecting an RCA plug to the XLR jack? I've seen a cable with XLR's at one end, RCA's at the other. Acceptable?


Yes any proper converter or cable that grounds pin 3 (signal -) is fine. I purposely made the amplifier perform the same either way. Balanced helps in situations where people have ground loops. 


@assetmgrs Roger; How do you feel about the design characteristics of hybrid amps, say with a tube preamp and a solid state power amp like the Rogue Pharaoh? Does this approach offer advantages of both tube and SS, or does it compromise the best of both sides? Thanks in advance.


So this would be a integrated. I looked at a picture. I would be concerned about tube noise. I have heard their tube phono stage is unusable due to noise. I see the line stage is a 12AU7, thats a poor choice as quiet ones are hard to find. Might be a problem on sensitive speakers.

I dont see a noise or distortion spec on their literature. Thats a little bothersome. The damping factor of >1000 is very hard to verify and will be cut in half with even a heavy speaker cable. So its kind of silly.

I worked on older Rogue amplifiers and was not impressed. Too many things done strange, like DC heaters for the output tubes and running them way too hot. When I see that kind of stuff I have to wonder. There was no way to reduce the idle current (bias) so I had to put in the most expensive output tubes to be able to guarantee any reasonable life.

I hope their new stuff is better and wish them success if it is. Keep in mind it is a value product, not necessarily high end.
Can you please tell me the purpose of resistors R1 -> R8 in the power supply shown below in the URL ?  Is it to reduce the resonant frequency in case the supply oscillates with the stray inductance of the smoothing caps ?

https://www.passdiy.com/project/articles/burning-amplifier-1

Thanks

@stereo5 

I have a question for you Roger:
What are your thoughts on Class D amps? I have listened to quite a few and always came away unimpressed with my ears aching. I am a Mac guy at heart marrying a Mac solid state amp with a Mac tube preamp. Perhaps I like a more mellower sound?

I heard your tube amps back in the 90’s at Sound II in North Dartmouth, Ma. Leo (RIP) was a huge fan of tubes and carried your line as well as VAC and Audible Illusions. I did a lot of listening and buying in that store. An added plus was there was a fantastic strip Joint right across the street from the store on Route 6. Really fun times!


I was making class D amps in 1990, before they were hardly known. Mac Turner brought me in to consult on his. What disturbes me about them is that they all get their sound from the IC in the front end. Perhaps some are discrete front ends but I have not seen any. 

The biggest problem is the output choke that is necessary to filter the switching waveform. If you look at Stereophile reports you will always seen one impedance that is flat while the others either peak or droop. 

They are going to be popular due to size, low cost, ridiculously high power that hardly anyone needs. I was interested back then as an engineering exercise but no desire to make any. Over 100 watts is only justified by either high listening levels or insensitive speakers or both together. Excess headroom is a myth. 

I have some Mac gear to be quite excellent. I have a post about that here. Just go to my profile to find it.


@cakyol
Can you please tell me the purpose of resistors R1 -> R8 in the power supply shown below in the URL ? Is it to reduce the resonant frequency in case the supply oscillates with the stray inductance of the smoothing caps ?https://www.passdiy.com/project/articles/burning-amplifier-1


They are part of the RC filter of the power supply, to reduce ripple. A choke would be better but expensive and bulky. He is likely using multiple resistors to spread the heat and often reduce cost.

It is nothing to do with resonance or stray inductance. Those arent problemsl. Its simple AC ripple filtering.

They could be replaced by a single power resistor of sufficient wattage at 1/4 the value. They appear to be 1 ohm each which is rather low to do much. Nelson is a good guy we spoke together at Burning Amp 2018. http://http//berkeleyhifischool.com/having-fun-at-burning-amp-2018/  

In his earlier presentations he talks about the Amp Camp. It is once again available at DIY audiostore. Its not my kind of circuit but it is interesting.
Roger, thank you again for your response. My ears have required long warm up times for almost all of the gear I have had over the years. Solid State as well as tubes, dacs, tuners, tape machines, etc. I suppose R. V. knew what I was talking about, the warm up problems. Is the forming switch the same as an amp having a stand by mode ? I assume it is this capacitor thing that I have been responding to. I am glad you are here posting, as I am greatly enjoying the information. Enjoy !


Glad you know your ears, many people dont. 

SS equipment can be very temperature sensitive because transistors themselves are much more sensitive than tubes as I have written here to another question. Therefore there may be a lot of variation over time. 

The forming switch is not like a standby switch. A standby switch, such as on a guitar amp, keeps the tube heaters on but cuts off the B+ to let the amp cool and extend tube life. However it is intended to be used by the player on breaks not longterm. Turns out long term, tubes do not like to have only their heaters lit without B+. Cathodes tend to get lazy. They learned this in the tube computers. So even overnight standby can start to be a problem and really has no sonic benefit. Standby on a Fender is so the player can go get a beer.

I designed my amplifiers to warm up to optimum pretty fast. I have worked on other amps, expecially ones with SS devices in the power supply, that take quite a while to come up to optimum.

I have had many responses, good questions, no Trolls. If I missed your question please re-ask it again.
Thanks for your expertise. I have a question about damping. You said a heavy gauge speaker wire would actually decrease damping. So a speaker gauge around 16 might exert more control over the woofers for deeper bass?
20 C/122F is nicely cool, only 25 C above ambient in a standard room. You either have big heat sinks or not so much idle current.

Do you mean resistors from 0.1 ohms to 0.02 ohms? I like the American system of resistors. That is a large range. Do your account for this and make the idle currents equal in each transistor?

HFE for a power transistor at low current is rather meaningless. Why not do all of it at operating current?

A VBE multiplier is a very good thing and easy to add. How much does your total idle current vary from cold to hot and how long does it take to get there? What is the idle dissipation?



Big heat sinks: 250 in2 , plus similar sized black anodized aluminum end pieces, per mono block.

I tried to indicate 0.08 ohm to 0.12 ohm. Mostly tighter around 0.1.

(Blush) Because it’s easier (blush)

Idle current diminishes from 1600mA to 1200 mA, in the course of one hour. Rails are +25V and -25V, so dissipation 80W reducing to 60W.

@blueranger

Thanks for your expertise. I have a question about damping. You said a heavy gauge speaker wire would actually decrease damping. So a speaker gauge around 16 might exert more control over the woofers for deeper bass?

What I wanted to convey that a damping factor of 1000 will never reach or be appreciated by your speaker. Above 10 is generally enough, hard enough to obtain on a tube amp. SS amps will have higher factors but to no avail at the listening end.


You want low resistance cables. For a few meters 16 ga is fine. Double the length and you should subtract 3 from the ga to get the same resistance, therefore 13 ga. Isn’t it interesting that every 3 ga doubles the resistance over the entire range of wire...Bet people dont know that one. You can only learn that at a traditional school like UVA. Any alumni here?

But here is the worst news to many is; Damping does not really control a woofer. Im sorry as this will disturb many. Paul Klipsch spent his lifetime trying to put this across. I dont know why it ever started being called damping. Perhaps someone here will do a little research. My clue would be to see when pentodes replaced the 45 triode in radios of the late 1930s.

We have to keep in mind that these radios and phonographs had single 8-12 inch speakers and an open back cabinet. In that case one could say we are damping the woofer by providing it a low impedance so that the resonant peak will not be so big.

Modern speakers dont require damping, they are already damped in their cabinet. They really are. Go tap on the woofer, you will hear the resonant bass frequency. Short the terminals and see how much your tap changes in level. I expect not much. You can even do this with your speaker connected. Tap it with the amp off then tap it with the amp on. The amp will be the short if it is high damping. its not quite fair to do this with a tube amp as the transformer may damp the speaker more with the amp off than on. HUM, isnt that interesting.

The resonant frequency of the woofer determines the low frequency cutoff and will not change with wire, damping or anyone's good wishes. It will however go a bit lower with time as the woofer loosens up, but only a few Hz. 

What modern speakers need is a low impedance drive to deal with their often widely varying impedance. A 8 ohm speaker can easily go from 50 ohms down to 4 or 3 or 2 ohms. That’s quite a range. Then damping makes a difference because it provides constant voltage to the varying impedance.

Of course some speaker makers keep their impedance rather constant, which is a very good thing if you want to play with a wide variety of amplifiers.

Maybe an open minded university, interested in electonics history, would let me do PhD research on such an arcane topic. Offers accepted. Though I hardly have the time for anything so rigorous.
@terry9  Idle current diminishes from 1600mA to 1200 mA, in the course of one hour. Rails are +25V and -25V, so dissipation 80W reducing to 60W.


Sounds like a nice amp. High idle, reasonable rails. Congrats.  Its for more sensible than some 200-1000 watt monster or a bridged amp.

I am puzzled why the idle current goes down rather than up as it warms. Usually its the other way round because of the negative temp coefficient of transistors.