SRPP 45 and 2A3
In general I prefer the simplicity and purity of a plate loaded voltage amplifier [R-C couple], however i found SRPP [shunt regulated push pull] sonically satisfying using low-gm, hi-mu tube like the 6SL7/5691. if you find the Simple 45/2A3 a bit slow or lacking in definition, then this may be your solution.
I do not have the technical background to fully explain how SRPP works however through pure logic, the top half of the triode acts like a constant current load to the plate of the bottom triode and the drive signal is basically a combination of the low impedance nature of the top half's cathode and voltage amplification ability of the lower half, Giving this input/driver circuit greater potential drive signal to the grid of the output power tube.
This is a simple and straightforward design and the only thing I need to point out is the voltage divider network in the power supply that lifts the cathode potential of the 6SL7/5691 around 70-90 volts above ground. Thus, the +/- 90V heater [filament] heater to cathode rating is not exceeded as per the RCA manual's recommendation.
How did you find the srpp 45 version vs the simple 45?
ReplyDeleteBack in the late 90s, I felt that the Simple 45 played more LPs and CDs whereas the SRPP was more audiophile oriented presenting more detail which on long term listening was not as pleasant. However, I had a chance to hear a Morikawa SE 2A3 owned by a friend - https://jelabs.blogspot.com/2014/01/altec-2-way-horn-system-redux.html - I enjoyed listening to this similar hi-mu triode/SRPP input amp! So YMMV!
DeleteJE
Thanks JE, I'll go with the simple 45 for now as I've been wanting to do it for awhile. I'd like to use a 6sf5 as a driver as I have them in my junk box. Would the existing circuit be ok or should I change the parameters to better match the 6sf5?
DeleteRitchie
The original circuit works well as-is with a 6SF5. But if you want to compensate for the slightly higher Rp, use a 150k plate load resistor instead of the 100k.
DeleteSounds good.
DeleteThanks very much.
I am using 105db sensitive horn speakers. Do you think ac on the filaments would have low enough hum to not be heard on this sensitive of a speaker?
DeleteI'll try the ac first, a lot easier to implement.
DeleteThanks again.
Ritchie
My Altec 2-way speakers are about 99dB/1W/1M and ~ 2mV of hum from my SE45, 46 and 2A3 (all 2.5VAC filaments) is hardly audible. If you get hum levels greater than that, there might be wiring/grounding issues.
DeleteI just don't know how that translates to a 105dB speakers.
Hello, these post seems to be inactive from some time, anyway, I have built this design some years ago. Used 5U4 for the rectifier and found than B+ reaches 290 volts and I am getting audible hum. Not a expert but I wonder why the first cap is only 2uF after the rectifier, that seems to produce a lot of ripple in my case and audible hum. What am I missing? Thanks.
ReplyDeleteThree years ago my late friend Steve of Angela Instruments sent the SRPP 2A3 shop mule I built for the in the 90s for a check up - https://jelabs.blogspot.com/2022/07/srpp-circuit-redux.html - that had a 2uf cap at the output of the 5U4 rectifier followed by a 10H choke and 50uf cap. I measured about 2mV of noise at the output.
DeleteThe only time I encountered objectionable noise issues (over 5mV) was when I used a 2uf (or less) at the rectifier output followed by a 5H choke. To quiet it down, I used a 100 or 150 Ω (?) 20W resistor at the rectifier output followed by a 20uf - 5H - 50uf.
You can also try to double or triple the capacitance after the 5H or 10H choke.
Thank you JE. I have already doubled the second cap, last time I measured ripple in B+ it was like 200mVpp. Used a simulation software and the ripple predicted by software was similar to mine. I am knowledgeable to some extent but I’m no expert and this has been my first and only build. What could I be missing? I was thinking about increasing the 2uF to say 20uF and placing a power zener to counter the B+ increase. Thank you very much.
ReplyDeleteI haven't ventured into computer simulations because I've been successful doing things the old school way for my own hifi pleasure. Since my formal training is in classical music, remote trouble shooting isn't my forte.
DeleteThat said, 200mV is waaay too noisy. Do what you can to lower it and if simulation software and/or increasing capacitance doesn't solve the issue, perhaps the noise is caused by something else - a ground loop, signal routing, etc.
Good luck!