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

@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.

@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.