• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Group buy pulse check for Pete Millett tube crossover PCB

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#43
Re: Re: initial observations

kevinkr said:


1. It's a VCVS based implementation, VCVS don't invert phase, (in band that is) however they do have some delay.

2. Something wrong here, as gain in band should be relatively close to unity figuring each stage has a gain of >0.9 inside the passband. I would expect something like -1dB to -1.5dB in band.

3. Gains of much less than expected will affect the transfer function of the filter and the X-O point will not be in the expected place nor of the expected magnitude.

I have built very similar X-O in the past and once you compensate for the fact that less than unity gain is available in the filter blocks you can get very reasonable approximations of any moderate Q transfer functions that can be realized at near unity gain.

What supply voltages are you using, and what is the load impedance you are driving? What are the values of the caps and resistors used in your filter, and are the resulting filter network impedances relatively high compared to the source impedance of the stages driving them?
Click to expand...



1. The delay may be large enough that I thought the signal was out of phase. I will go back and check.
2. If I were looking at the gain off of the cathode, I would expect slightly <1, but now you add all of the passive components.

For a 600Hz cutoff, the C is 0.001uF and R is 185K. Between stages, the signal is running thru 2R.

Voltage rails are +/- 90vdc, load is 100K
 
#44
Re: Re: Re: initial observations

audio_builder said:




1. The delay may be large enough that I thought the signal was out of phase. I will go back and check.
2. If I were looking at the gain off of the cathode, I would expect slightly <1, but now you add all of the passive components.

For a 600Hz cutoff, the C is 0.001uF and R is 185K. Between stages, the signal is running thru 2R.

Voltage rails are +/- 90vdc, load is 100K
Click to expand...

I think something is wrong though - did you build it as shown or succumb to the temptation to self bias the CFs? Sounds like you didn't, but check for swapped pins on the tube sockets and other embarrassing sorts of errors of the nature I make all of the time.. 😀 The only resistors present at the input of each filter stage should be those two series resistors in the filter network.

Those 2 resistors are part of the network that realizes the filter function, inside the passband they produce little or no attenuation.. (but those large values will contribute quite a lot of noise) Given the low x-o frequency you can probably increase the capacitance by a factor of 5X and R becomes 1/5R which is better from a noise standpoint and can still be driven by a 12AU7.

I have built active tube based Butterworth filters and had them behave exactly as spice predicted. A Linkwitz Riley implementation should also work pretty well. (I'm just not a fan of higher order filters.)

I'm not a big fan of the 12AU7A, but the 12BH7A can be pressed into service at higher currents, and the very linear and good sounding 6FQ7 will work with a minor hack to the filament pins. (cut the trace between pins 4 & 5 and apply 6.3V here)

An even better choice for the more ambitious would be the 5687, which has higher transconductance and lower rp than the others and should make a better CF than the ones I've listed above. (Slightly closer to ideal performance with the vcvs, and allowing scaling of component values to improve noise performance.)
 
#60
when I type www.sinofpcb.com into Google I get the below.

"Sorry, 'www.sinofpcb.com' does not exist or is not available."


I also went to the initial website where the DIY project was posted an sent Pete an email, yesterday and a few times over the last year, prior attempts not responded to.

If someone has some verifiable contact info, that would be great.
 
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