March, 2018 SFAS – An Afternoon with Roger Modjeski
Analog and Audio Integration
At the Hilltop
March 10, 2018
The SFAS event for March was held at The Shops at Hilltop in Richmond featuring Roger Modjeski. This was the first event at held at Hilltop which fulfills the needs of having events dispersed around the Bay Area plus relieving some of the pressure on Leslie’s audiophile garage. We’d love to hear your thoughts on this location for SFAS events.
The following is a quick introduction for those of you who may not be familiar with Roger Modjeski. Roger is the founder of Music Reference electronics and RAM Tubes fame ( http://www.ramlabs-musicreference.com/ ). The Music Reference RM 200 power amplifier is one of the amplifiers Roger is famous for designing and it has been included in Stereophile magazine as a Class A pick for many years. RAM tubes have the distinction of being some of the quietest tubes one can purchase. Additionally, Roger has established the Berkeley Hi-Fi School to broaden learning, knowledge and truth in audio electronics (http://berkeleyhifischool.com/ ). Roger has participated in at least two SFAS events in the past that I’m aware of: a phono stage shoot out and a tube and amplifier discussion, both of which were very well received by the SFAS membership.
The March event, as the title of the event “Optimizing your phono/tape and Complete System” indicates is primarily focused on lowering the noise floor and reducing hum in phono, tape and entire HiFi systems. Roger was kind enough to develop and distribute a step by step process to follow to help achieve these goals. The following is a link to the “Optimizing” document for your use and reference (Optimizing Your Phono).
Roger began the session discussing tubes as a noise source. Yes, the tubes we all love and love to hate can be a source of noise when used in the front end of an amplifier and if there’s noise at the front end it will only become amplified. Although Roger gets noise to the theoretical limit, tubes can become noisy over time. Solid state front ends have noise but it will be more likely to stay fixed through the life of the amplifier. So it probably isn’t surprising that Roger designed his hybrid RM 200 amplifier with a solid state front end and tubes for the driver and output stages. The input device is an ultramatched transistor pair which gives far better common mode rejection and provides the same performance whether driven balanced or unbalanced (single ended).
Roger talked about how each step in the audio chain from source to power amplifier not only amplifies the signal, but will also amplify any noise. Additionally some equipment may actually have such a large amount of voltage gain available it can exacerbate the noise. If one does have such a situation where the voltage gain is too high Roger suggests using an attenuator between the preamp and amplifier. In many cases the attenuation needs to be 10 to 20 dB.
Roger then talked about the new phono stage he’s designing and building. A unique element of this phono stage is that the gain of the amplifier is designed to match the RIAA curve to keep the feedback and distortion lower by maintaining the same amount of feedback over the frequency response. See the stepped RIAA curve on the white board in the picture below.
Additional topics Roger discussed are as follows:
- Active vs. passive RIAA equalization. Active equalization with tubes can be problematic as the gain will change (decrease) over time which will exhibit as a decrease in base response. Passive equalization can have its own issues with overload and noise.
- Cartridge loading – many cartridges don’t respond to loading – depends on the impedance of the cartridge – MM cartridges have high impedance and the impedance of MC cartridges can vary by a large degree. The important thing is to follow the guidelines specified by the cartridge manufacturer and don’t overload a cartridge.
- Comments on tubes and tube rolling – Mu (voltage gain), Gm (transconductance) and Rp (plate resistance) are the three electrical characteristics that make up the overall electrical characteristics for a vacuum tube. The differences in sound quality perceived between different tubes may be attributed to differences to gain by the tubes. For more information on this topic please see the following link to an article written by Roger: (http://www.tubeaudiostore.com/tubmatdem.html).
The following are some hum/noise reduction tips discussed during the event, some of which are covered in the handout:
- Mono blocks can be prone to problems with hum usually caused by a ground loop – one solution is to float the ground on one of the amps.
- If you have hum in the system, ground the amp and float everything else.
- Avoid making loops with ICs – bring them together and ziptie them together. Also try moving the cords around to find a location with the least amount of hum or noise.
- Shorting plugs and an oscilloscope are good tools to have on hand. Don’t test with an open input.
- A lot of extra headroom (power) on an amplifier is generally not needed and can be a source of extra noise. Only need extra power for low sensitivity speakers.
Some new products Roger is working on are a tube tester, a tube curve tracer and an integrated amplifier. We’ll look forward to hearing more about these in the future and SFAS will be looking into purchasing a tube tester for use by the membership!
All the SFAS participants appeared to enjoy the event immensely and found it both interesting and informative.
Roger wanted to put out the announcement that he will soon be moving his entire back-stock of tubes from a storage facility to his new location. Any helpers would be appreciated. For their participation he would give them an in-depth class on tubes and amplifiers. You may reach Roger at: firstname.lastname@example.org