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.


128x128ramtubes
clio09
3,360 posts
 
@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.
krelldreams
128 posts
 
@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.

ramtubes's avatar
ramtubes OP
460 posts
 
@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. 
clio09
3,360 posts
 
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.
ramtubes's avatar
ramtubes OP
460 posts
 
@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. 
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