Gio;
As to the channel to channel architecture, there are two ways to go. The first is obviously to use one tube for L+R preamps and one tube for L+R buffer. The advantages in this case are the ability to use different preamp tubes. The disadvantage is the potential for some low level crosstalk between channels. However, I believe that if the B+ supply is split with separate final filters for left and right channels then you shouldn’t get any channel crosstalk; but that’s just my guess as I haven’t tried to measure it. The second choice is to do as you suggest and go with 12AU7s and use one tube (preamp + buffer) for each channel. The advantage is that with split supply this virtually guarantees near zero crosstalk. The disadvantage is that the only gain selection you can do is by bypassing or not the cathodes of the preamps (≈+/-6dBv). You could go either way. My guess is that you probably couldn’t tell the difference.
I have designed a tone control preamp I’m going to build and have done this same trade in my head about three times. I can’t decide on which way to go, but I am slowly going to the separate tubes for preamps and buffers approach. This is mostly because I can’t force myself to give up the versatility of the 4S preamp architecture for swapping tubes.
The choice for the volume control is a little more problematic. Again there are two options. The first is exactly as I have drawn the circuit with the volume pot on the input. The advantage of this design is that the potentiometer only handles low level ac signal. The downside is it’s noise figure dominates the preamp. The other option is to replace the load resistor of the preamp stage with a 100kΩ potentiometer, the wiper of which is connected to the buffer stage through the 0.1µf coupling capacitor. The advantage here is that the noise figure of the control is now integrated with that of the preamp (i.e. it should be better than the control in front of a preamp stage) and control noise (if any) is no longer in the grid circuit so it is not amplified by the preamp. But there are a few disadvantages. First, whereas the noise figure is better than the first option, the placement of what is likely a carbon load on the preamp raises the noise figure of the stage significantly from what it would be with a metal film resistor load. This means that the noise figure improvement from moving the gain control is not nearly equal to the gain of the preamp. The second is that the potentiometer will now be operated with a significant DC bias (≈300v). This means that the potentiometer must be made to handle this with proper isolation. The PEC KK series will do. They have high insulation resistance, 900vRMS dielectric strength at sea level, and an operating voltage range of 500v. One like this should work nicely (
http://www.digikey.com/product-detail/e ... -ND/770961) but it is a little pricy at $31.26 USD.
The only other unknown is the output coupling capacitor. Using my 5.3Hz rule with the 100kΩ amp input impedance, the calculated value is 0.41µf; A 0.47µf value should work just fine. If you are driving a lower input impedance amp, the value will have to grow to keep the low end from suffering.
I realize that I’ve answered your questions with more options. But so much of these types of build decisions rotate around such trade offs that I really have done about the best I can to answer your questions. Read it over a few times and let me know if you have any further questions.
Regards.
Matt
Login
Register
FAQ
