2. amps-preamps
  3. 140029-80294

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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georgehifi's avatar
georgehifi
8,555 posts
krelldreams
my passive across the input to the output as you requested. It measures 1.21kohm, which is as written on the bottom of the case.
If your Alps pot’s passive pre, input to output measures 1.21kohm then you have a 5kohm Alps pot.
Your source may well be doing it hard into this 5k load, and making things "leaner".
Passives should be at least 10kohm to suit "most" situations. Or higher (>10kohm) if the amp is also >50kohm or more.

Cheers George
ieales's avatar
ieales
756 posts
Passives should be at least 10kohm to suit "most" situations
It depends on the source output impedance what the passive resistance should be. If the output impedance is < 500Ω, a 5k pot is fine.

Pots have a different sound than stepped resistor controls. IMO, P&G pots were more musical than Alps when I used passive level controls in the studio. Additionally pots, especially log, track nowhere near as well as stepped resistors.

Clio wrote about a phono preamp with passive level control and aux inputs. What a great idea!
Whether it is a great idea depends on the topology and to what it is connected.
krelldreams
52 posts
Thanks George. I like the idea of a passive control device, and I’m trying to learn about the system matching aspect so I can try an appropriate unit. I did appreciate the clarity, but was distracted by what I’ll call an overly etched, and leaner presentation. I can accept the fact that there may be more going on here than the “passive vs active” thing. The only passive I have was ordered for a very different application... level control for a CD player into a vintage tube amp for a second system. To be honest, it sounded pretty good despite the fact that it was MUCH less expensive than my preamp (1/10 the cost). Mr. Modjeski seems to endorse passives, and I’d like to at least explore this avenue with the right unit (for my system). My power amp is a CJ Evolution 2000 which was upgraded/updated by Bill Thalmann of Music Technology (formerly of CJ). Its input impedance is 100k ohms, and input sensitivity is 900 mV to full output. I’d like some flexibility with whatever I choose as a source switch/level control in case I decide to use a different power amp. 
ieales's avatar
ieales
756 posts
I’d like some flexibility with whatever I choose as a source switch/level control in case I decide to use a different power amp.
See http://ielogical.com/assets/Audio/PassiveLC1.png here http://ielogical.com/Audio/#LastHurrah. It is possible to use a single resistor value to give perfect passive tracking for any amp impedance. Only caveat is ensure source has low output impedance, say less than 500R and power amp has reasonably high input, say 20k+
georgehifi's avatar
georgehifi
8,555 posts
Mr. Modjeski seems to endorse passives
Yes his amps are loved by passives and with my customers that use them.

My power amp is a CJ Evolution 2000 its input impedance is 100k ohms, and input sensitivity is 900 mV to full output.
It should also love passives, with that 0.9v input sensitivity. And that 100k input impedance you could of had a 20k Alps passive, not just 5k,

A 20k Alps pot would have been fine for just about any source then, which by the way you have not told what you have as the source.?? because if it has high’ish output impedance it won’t be comfortable with that 5kohm Alps pot as it will load down it's output and also create a voltage divider.

Cheers George
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