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.
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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.


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@bdp24. 

 I’m thinking that may be exactly the avenue I’d like to explore further. I’m looking at phono preamp options as we speak. I have a very simple passive device, which I did try, but didn’t care for. I am very willing to try other passives though, since the one I have was purchased to provide level control for our kitchen system, where I was using a 1960s HH Scott tube power amp.

 The unit is a Luminous Audio Axiom. When I ordered it they asked what amp it would be used with so they could make one to work specifically with That amp. It is entirely possible that it is a mismatch with my primary amplifier.

 Roger asked for specs... the only spec I know of is “1.2k” hand written on the bottom of the case. This was only a test anyway since I’d need more than the single input this one provides. I was not aware that Music Reference made passive devices (?). 

 I am considering both tubed and solid state phono preamps. Any design parameters that I should be paying attention to? Any designs to avoid? I currently only need one for a high-output MC cartridge (MM gain and loading), but I plan on trying other cartridges in time as well. I’m not opposed to getting a step up device if a cartridge I get in the future needs one. 

Thanks again for the advice!
@bdp24 - technically no, Roger does not offer a new production phono preamp (although we have some unique RM 4+ units lying around that in addition to phono include a volume control and inputs for other sources as well). I have a tape head/phono preamp that he designed in my system right now and which we showed at a SFAS meeting a few months back. It is a 2 box unit with separate power supply. It’s not an easy build and getting it mass produced will be tough, but if someone really wants one built to order, just phono only, and is willing to wait while it is built they can email Roger or I at tubeaudiostore@gmail.com to discuss.

Pot in the box passive preamps can still be made to order. It’s an easy build and Roger has plenty of the really nice Noble pots available that he likes.
@clio09. I believe bdp24 was suggesting I consider having Roger build me a passive (for switching and level control), not a phono preamp. In addition to an input for phono, I’d like to have at least two other inputs, and preferably two outputs as well.

I searched RM4+ and found only RM4, which appears to be a MC phono preamp only (?). If there is such a thing as a phono preamp which adds a couple of aux inputs for other sources, and a level control, I’m all ears. I’m not familiar with the “pot in the box”. Would you provide some more details as to its design? 

Also, in your opinion, from a designer’s standpoint, what are some some things to look for, and to avoid in choosing a quality phono preamp? Thank you.

@ieales  I'd be curious if trained listeners can reliably detect the sound of the switcher



Trained listeners have found the switcher to be transparent. Sounds like in your current setup the listener does not know which he is hearing at the moment? We could certainly go to that. With the results we are getting the difference is easy to pick up on. 
I probably should have put this in quotes from one of bdp24's posts to eliminate any confusion:

Roger Modjeski (@ramtubes) prefers the 6DJ8 for pre-amp use, but I don’t know that he currently offers a phono amp. Roger or clio09?
That was what I was responding to. In another post bdp24 did mention Roger building a passive for you. As I mentioned in my last post the Pot in the Box passive attenuator is available. The design is very simple using a Noble pot that is laser trimmed, thick film, discrete steps. They are the same extremely high quality Noble pots used on the Audio Research SP-6 and the Beveridge RM-1 (which I own). Typically these are built with two inputs with a selector switch. The front switch also has a mute position (middle position). If you are interested in some specs:

Input Impedance: 50K Ohms
Output Impedance: 0 - 25K Ohms
Frequency Response: DC - 100K Hz

The RM-4 was offered in two formats. The first a pre preamplifier, better known as a moving coil head amp for MC cartridges. Then there is the RM-4+ that adds an RIAA circuit, with optional attenuator and inputs for other sources. We have a couple of these lying around the shop. IIRC correctly one has 2 inputs and the other 6, both have an attenuator and input selector switch. You can send an email to tubeaudiostore@gmail.com if interested.