 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
75.168.141.235
In Reply to: RE: Noise/hum posted by Tre' on January 06, 2025 at 07:51:21
If so I see some real problems.
The first is the cathodes of the input tubes should be tied together. Because they are not there is zero Common Mode Rejection. It relies entirely on the input signals being out of phase. So if the audio cable passes thru a hum field (like its near a power transformer) the noise impinged on the cable can get amplified.
To do that properly there should be a constant current source in the cathode circuit. Otherwise the CMRR will be poor instead of non-existent. Because of the Voltages involved there should be a minus supply for the CCS circuit. 6.3V would be plenty if the CCS were properly designed.
The volume control at the output is problematic. Its purpose is to gain ride noise. But it also increases the output impedance and the 2.7K resistor to ground makes no sense; depending on the volume control position the output section is driving a 17.7K load!
If the 15K resistor and the 2.7K resistors were removed, the former replaced by wire and the latter with nothing, the output impedance would be lower and the 0.47uf caps would make more sense.
There is enough gain available from the first stage that the output tubes could simply be cathode followers. The parts count would be lower and less tubes used. Probably lower noise too.
However both the inverting and non-inverting outputs reference ground so ground loops can be introduced. That would not be fixed with cathode follower outputs.
To fix this using an output transformer would not be a bad idea. That might dispense would a lot of tubes! One designed to drive a 600 Ohm load (with a 600 Ohm resistor available with a switch) would allow the circuit to drive any load out there (if not 600 Ohms the resistor would be used so to keep the output of the transformer properly loaded to prevent ringing).
This is all assuming the schematic is the real thing... all IMO of course.
Isn't this a type of cathode follower?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Totem Pole OTLs like the Futterman look a little like this.
Looks sort of like an SRPP circuit. But if so it's performance could be improved by a better CCS for the bottom tube.
I would have considered making this differential with the other phase output. But I would have the input circuit set up as differential as well- everything to increase the CMRR. By doing so, even ordered harmonics are cancelled, which results in a cubic non-linearity rather than a quadratic non-linearity. In this way, distortion harmonics fall off at a higher rate since distortion is compounded less from stage to stage. Since the 3rd is treated by the ear much the same way as the 2nd (in that its innocuous) the 'sound' would still be smooth, as with no feedback the inherent distortion would be a significant 3rd harmonic which could easily mask succeeding orders.
So IMO/IME this circuit leaves a fair bit of performance on the table.
Isn't that a White cathode follower? Yes, the two sections act in push-pull.
Edits: 01/07/25 01/07/25
-
I'm assuming the resistor between the upper cathode and the lower anode is a fairly low value. It's not unusual to "build out" a follower with 100Ω or so for stability. Only Tre' knows for sure. :)
I simply redrew the schematic from the preamp that is the subject of this thread. That resistor is 150 ohms and the output cap is too small at .47ufI left out the 330 ohm "plate resistor" for the top triode. Without it there would be no signal for the bottom triode. Oops.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/08/25 01/08/25
With those values, I'm confident this qualifies as a White cathode follower. I don't believe the addition of a 150Ω resistor in series with the upper cathode has a significant impact on the circuit's operation. Easy enough to confirm in SPICE, but I don't think it's necessary.
Why on earth that coupling cap is such a low value!
I think if I were tinkering with something like that I'd have tried to use a common cathode resistor or a CCS, just to get as much differential effect out of the output as I could.
but it's not clear to me from the print exactly what impedance it's feeding. There's a 15KΩ, then a stepped attenuator and finally a transformer. :/
If you leave the transformer out (it's only there for a unbalanced output) then you're left with the 15k in series with (2.7k//whatever the input impedance of the amp is). If we call that 2.7k then, with the attenuator turned all the way up, the .47 cap is playing into 17.7k. 2nd edit, Ralph already said this.That gives a -3db point of 19.13Hz. That would still be 1db down at 38.26Hz and there would be phase shift all the way up to 191Hz. Turning the attenuator down would make it better but having a high pass filter, in the audio band, that changes with the volume control setting is bad form.
I mis-spoke. That's not the volume control, it's the trim. So it's not as bad as I thought.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/09/25 01/09/25
.
Have Fun and Enjoy the Music
"Still Working the Problem"
That schematic is from the owners manual provided by the maker of the preamp. See link.Also, here is the link to their page about the preamp. Scroll almost to the bottom of the page for the link to the owners manual that includes the schematic.
https://www.decwareproducts.com/ztpreTre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/06/25 01/06/25
I've probably said too much already...
!
The Mind has No Firewall~ U.S. Army War College.
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
