It looks like the best way to use that gain reduction method would be to start with a high mu triode like 12AX7 and put a source follower on its output to drive the inputs of today's solid state power amps with 10k ohm volume controls, etc.
It doesn't look like that method works so well with a low mu triode like 12B4A.
It doesn't look like that method works so well with a low mu triode like 12B4A.
I have riaa/streamer/reel tape out going into autoformer volume control and then into amp. The tubelab SPP el84 is ultrasensitive, the 300b less so but still way too much gain to need a preamp.
Revisting this.
Maybe the better way would be a transformer output to step down.
My pre is based on Eli's design.
I need to learn how to select the proper opt .
Maybe the better way would be a transformer output to step down.
My pre is based on Eli's design.
I need to learn how to select the proper opt .
The problem with a stepdown transformer on the output is that the tube will now have to amplify more just to have that gain thrown away by the transformer stepdown ratio.
Since more gain always means more distortion, you've basically increased the distortion at the same output (volume) level.
People usually use an OPT on a preamp to either get that 'transformer sound' or to split phase (convert unbalanced output to balanced outputs).
At 12B4A with 80V plate-cathode and 20mA cathode current should yield mu of about 6 and Zout of about 1.2k ohms.
If you configure your CCS in the 12B4A plate to be a mu follower you can take the audio signal output from the CCS source (if using a MOSFET), which will give you very low Zout.
I ran this circuit simulation
It says the circuit will have about 6.5X gain, and with 1Vrms out at 1kHz, THD will be about 0.07%, almost all second harmonic.
Not super hi-fi (the THD is kind of high) but the Zout is nice and low at only about 100 ohms.
--
Since more gain always means more distortion, you've basically increased the distortion at the same output (volume) level.
People usually use an OPT on a preamp to either get that 'transformer sound' or to split phase (convert unbalanced output to balanced outputs).
At 12B4A with 80V plate-cathode and 20mA cathode current should yield mu of about 6 and Zout of about 1.2k ohms.
If you configure your CCS in the 12B4A plate to be a mu follower you can take the audio signal output from the CCS source (if using a MOSFET), which will give you very low Zout.
I ran this circuit simulation
It says the circuit will have about 6.5X gain, and with 1Vrms out at 1kHz, THD will be about 0.07%, almost all second harmonic.
Not super hi-fi (the THD is kind of high) but the Zout is nice and low at only about 100 ohms.
--
I have ruled out transformers.
A friend of mine sent me a info on a nested loop feedback that reduces gain and distortion.
I will give it a try, it's a few dollars in resistors and caps vs several hundred for Xformers.
A friend of mine sent me a info on a nested loop feedback that reduces gain and distortion.
I will give it a try, it's a few dollars in resistors and caps vs several hundred for Xformers.
I use Harris line input attenuators for high output devices to temper the input signal levels. You can get 6dB or 12dB versions. @$25 but they are easy in easy out devices when you want to change it up.
That sounds like the good old "anode follower" - simple plate to grid local negative feedback around a single triode stage, like this:
R1 and R2 form a voltage divider feeding a portion of the output (from the triode's plate, with DC blocked by the output capacitor, to the input (triode's grid). That certainly does work. It will work better for driving low impedance inputs (like 10k ohms) if you choose a triode that already has fairly low plate resistance, relatively high plate current, and reasonably high mu, such as a 6DJ8/ECC88 (mu of about 33) or maybe 6N6P (mu of about 20).
Bear in mind that the value of R1 pretty much defines the Zin of the circuit. If the Zin is very high (100k or higher) it's likely there will be some attenuation of the high frequencies up around 20kHz. To avoid that you'll want to reduce the value of R1 to about 47k ohms, but now the Zin will be roughly 45k ohms, so if you have a vacuum tube source like a tube phone preamp then you'll need to make sure that source can drive 45k ohms without loading down. If your phone preamp has a cathode follower on its output then it should be fine. But if you're using something like the RCA manual phono preamp or the Dynaco phono circuit from the PAS2/PAS3 using 12AX7s then it won't be OK.
Also, that R1 series resistor adds whatever noise it makes to the signal received at the grid of the triode. However, I haven't found that to be an issue at line level (as in a line level preamp).
The 12B4A has low mu (about 6) which means if you apply 9dB of negative feedback to reduce gain by 3X, you can only reduce the Zout by 3X. For 12B4A, that means the gain goes down to 2X (6dB) and the Zout goes down to about 400 ohms. THD should also be reduced by 3X, which in this case would still be high enough to yield some of that 'tube sound' (assuming that's desirable).
If you were to use a 6N6P with in-circuit open loop gain of about 15, you could apply 16dB of NFB to reduce gain, THD and Zout by roughly 6X, Since the Zout of a 6N6P with 15mA of plate current is about 1800 ohms, that should bring the Zout down to about 300 ohms. THD should then be quite low.
Last edited:
The 12dB version says it's good for DC to 1MHz. but there is no internal diagram with them. Not sure if it's on their web site, maybe. So I couldn't say what components are in them. I haven't done any sweeps with them in place, just use them to lower 2v DVD outputs to standard inputs for volume knob sensitivity. But they seem flat to me, without a doubt.