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One capacitor and one potentiometer is all that is needed. The potentiometer is adjusted until the greatest null in output noise is reached. Which tube to use? I would look into using a triode connected EF86 or even a 6AC7, although a paralleled 12BZ7 might be the best choice.
As for your phono preamp, I would try eliminating the cathode resistors and use a 1.5 volt battery to bias all the 12AX7s. This will require one more coupling capacitor in the signal path, but it would also greatly lower the power supply noise at the output and increase the gain, while lowering the output impedance. This means the first resistor in the passive RIAA equalization network will have to be correspondingly increased in value.
Subject: SRPP
Hi , I was just looking at the May 2000 article on the SRPP circuit. Early on you wrote. "Just what "SRPP" means is uncertain; maybe it stands for Series Regulated Push-Pull amplifier or Single-Ended Reflexive Push-Pull amplifier." Recently I have had the good fortune to be able to talk to a few "old timers", and, whenever this circuit comes up in conversation they always refer to it as shunt regulated push pull. (The other day I saw some notes on the web by a retired Electrical Engineering professor who referred to it the same way.)
Funnily enough, when you first say "SRPP" they don't make the connection .
Hope this is of some interest to you. Your articles are of big time interest to a lot of people . Keep up the great work !!!!!
Thanks for the information. I am surprised that I hadn't listed that variation. I have seen many names for this circuit: SEPP, totem pole, cathode follower cascode, and cascoded cathode follower.
In the 1943 patent, the following sentence appears: "A further object is to provide a current amplifying means which acts as its own regulator of its own power source." This must be where the "regulated" entered the SRPP.
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Subject: preamp circuit
First, thanks for the great journal. It always gives me great pleasure just trying to understand what is going on. Despite the fact that my knowledge is limited, it hasn't stopped me from building many projects....and I continue to learn.
I am going to try a preamp circuit next....I've bought your Tube CAD software, and in reading the journal I've come across 2 circuits that look interesting: 1) your "ultra-linear" type preamp using a CF output stage and 2) your common cathode design.
I know you don't play favorites with circuits, but could you tell me which one I should rather build and why? Of course, we are all limited in the number of projects we can undertake....and I'm trying to obviously get the best sonics. I will probably go whole hog and transformer couple the output dependent on the price.
Dan D
Dan, it all depends on the purpose for the preamp. If the intended use is a line stage amplifier, then I recommend the common cathode amplifier. It does not invert the phase. It readily accepts a feedback loop. It offers a very high input impedance and a very wide bandwidth.
On the other hand, the ultra-linear cascode offers an very high gain without usual accompanying high distortion. This circuit would prove useful in a phono or microphone preamp. If used as a line stage amplifier, the gain will probably prove to high, although this will depend on the tube used. Transformer coupling this circuit with a step-down output transformer will help eat up much of the gain and greatly reduce the output impedance.
Subject: B+ Supply.
I really enjoy reading your E-Journal. Thanks for a great article on Hybrid amps. Though I am still not decided which way to go, I found this article very helpful in making a lot of things more clear. [Last issue's article on hybrids. Ed.]
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