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

@lemonhaze Hi Roger,Am I glad I found this helpful thread. I have a Mark Levinson 23.5 amp that hums.The hum is loud and stays constant in amplitude regardless of the position of the volume control. I am driving it with an Aikido linestage that I built and which from driving various power amps I know to be quiet. I tried lifting the earth leg first on the line amp and then on the power amp but hum stubbornly remains.  I bought the amp used. I realise that it is quite ancient and probably needs to be re-capped. I took a look inside and it appears somewhat daunting.

Has anyone suggested putting shorting plugs into the inputs of the amp. That is the true test. If the amp hums through the speakers just as loudly then it off to the shop. Nothing is lower noise than shorting plugs :)

Quality amps, like yours can keep their caps for 40 years if it runs cool enough to keep your hand on continuously. Most re-capping is performed by the shotgun method. They just do them all rather than find out whats needed. Perhaps thats not bad, but many times caps arent even the problem and what they put in may not last as long. Re capping has become way overdone. 

@whitestix  Roger,
I recall you saying at Burning Amp a year ago that you did not like 6SN7 tubes in preamps.  I have a preamp that uses these tubes and it is the best I have ever heard in my system.  What is the problem with these tubes from a designer's standpoint?  Thank you.

 Have a look at the most recent BA video where i discuss the tube. I point out where the mu starts falling off, its 5 or 10 ma. but have a look for yourself. Id like everybody to look at whatever suggestion applies to them. So look it up. 

As long as you can get ones that dont make noise or ring much when you tap them then you are ok. In my experience those are hard to find. Remember the 6SL7 was made for audio.The 6SN7 was made for black and white TV and was never, to my knowledge, used in the audio chain. When factories make a tube they pay attention mostly to the characterestics the tube was designed for, again not for audio, therefore they cared not about noise of microphonics. WIth 6SL7s they cared plenty. 

My guess is that people use 6SN7s simply because they have the right gain,  just by luck. BTW, the 6CG7. 6FQ7, 6GU7 was the 9 pin replacement for the 6SN7. Much easier to find. more modern construction. How do people like those?

YOU have to keep in mind the train has been a rolling for a long time on the 6SN7 for audio so it had a good hand on that tube for no reason technically. It also might be that this tube was just lying around everywhere with new old stock for days. People used to throw them away because after B&W televsion was replaced by color that tube was replaced  by the  6FQ7... right away. 

whitestix
I recall you saying at Burning Amp a year ago that you did not like 6SN7 tubes in preamps. I have a preamp that uses these tubes and it is the best I have ever heard in my system.

I have a friend that used these in his fully glitzed out Supratek Cabernet Dual preamp.
http://www.supratek.com.au/uploads/1/2/2/5/122515010/published/corttop2.jpg?1539764225
An he thought the world of them, till I went to hear what he was on about, as soon as I heard it, I said "that euphonics you say your hearing, I’ll show what it is, stop the cd and leave the volume where it is"
I sat him down in the listening seat and I gently taped the chassis of the preamp and there was a nice solid loud "donk donk" out of the speakers.
Your hearing "microphonics not euphonics" I said to him, then I very gently taped the actual tubes with my finger nail, and it was even louder with added "tinkle" to the "donk donk". Say no more.

Cheers George

@pryso

That is a simple A-B is not adequate, even though you may find differences. I believe at a minimum you need to have A-B-A.

What exactly is A-B-A? we just go back an forth as desired. All connections are cleaned with De-oix from the start.

@bifwynne  Not sure what you feel is deficient with the ARC Ref 150SE specs. If you care to elaborate, please do

Thanks for not getting at me since you own these amps. All I am saying is that Bill was on a mad quest for low distortion and high damping and achieved that in the one amp I called out for that. My only negative was its complexity. See if you can find that post. 

What I feel is deficient is exactly what JA thinks. Somtimes us folks do agree.  https://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements. What interesting to me is that most audiophiles agree with each other by disagreeing with the technical people. It is still a mystery to me why people without technical knowledge want to disagree with technical people who are making this stuff (for better or worse). Sometimes I feel like im just fueling the fire.

JA does me a great favor when he tests an amp, then I dont have to. I have 400 pages of test notes on 200 amps and preamps that I have measured over the past 35 years. Testing an amp is a laborious task if you really want to get into whats going on. 

For you and everyone out there. There are things going on in certain combinations of amps and speakers that cannot be easily predicted. However these effects are rarely present in a good amp with good performance measurements. I have tricks that go beyond what JA does. For instance I put a 0.1 uf cap (similar to some speaker cables) across the output of a big Adcom amp while was on the bench and luckily connected to an AC Line amp meter. When i touched the cap against the output terminals the 10 amp meter pegged and i quickly disconnected the cap. I just holding it. Didnt get hot though 1200 watts was going somewhere and that somewhere was the transistors. 

I know some people have unknown Birdies going on in there system. Birdies are brief supersonic oscillations that occurr on specific parts of the wave at specific levels. If you drive a simple woofer directly that is the easiest way to see them and they sound like clipping but you are not clipping. 

I have personally, in my own store in the 1980s, watched a BIG Levinson ampifier smoke when connected to a high end speaker cable that had some parameter (perhaps capacitance) that the amp didnt like. It just sat there as little trails of smoke came out, and frankly I was amused.

I know im off topic but... motto is. Just because an amp passes the 2 uF load test (somewhat standard) does not mean it is stable into smaller capacitances that might be present in many cables. Whether you like lamp cord or not, It is the proper constructin for a speaker cable. Just not exotic enough for most. Hey man, its just wire.

Anyone else smoked and amp with a speaker cable?