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In Reply to: RE: Seeking comments on Decware balanced preamp posted by SuperB on January 03, 2025 at 07:07:46
From everything on their webpage this preamp appears balanced but does not support AES48, the balanced line standard.
AES48 is important for two reasons. The first is to prevent ground loops and the second to prevent coloration by the interconnect cables.
The second aspect works in tandem with a second set of standards, -4dB and +10dBm. These are VU readings when the preamp is driving a 600 Ohm load and are thus showing how much power the preamp can make into 600 Ohms.
This preamp, like many in high end audio, makes the non-inverting output (pin 2 of the XLR) with respect to ground (pin 1). The pin 3 output (inverting) is done the same way. So the ground loop immunity is lost.
It cannot drive 600 Ohms properly. The 600 Ohm output impedance suggests it can drive 6000 Ohms but that depends on the value of the output coupling capacitors.
However, this is similar to most of the 'high end audio' balanced line preamps out there whose designers don't seem to be aware of AES48 or are intentionally ignoring it. So these comments above apply to those preamps too. I've only seen a small number of tube preamps that support AES48. Most of them have transformer coupled outputs (which is how it was done in the old days pre-transistor).
There are a couple that are direct-coupled.
I imagine that as long as you don't try to run long cables or have any problems with ground loops this preamp will sound just fine.
Follow Ups:
Ralph, I noticed the specs for that preamp list noise/hum at -58dB. Would you consider that adequate for a balanced preamp? Just wondering, because I worked hard on this issue in my own preamp.Also, do you think the manufacturer really means 600Ω output impedance, not 600Ω load?
Edits: 01/05/25
The specs say output impedance 600 Ohms. So it really should not be driving anything less than 6000 Ohms.
If you are driving horn speakers this seems like it would be a problem.
That's what I was thinking. I fought 60Hz noise in my new preamp for several days, but finally pushed it down to -80dB or better in both channels. I don't think the spec quoted by this manufacturer would be acceptable even in my non-horn system.
page 13 has the schematic. The .47uf output caps don't look big enough to me.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/06/25 01/06/25 01/06/25
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.
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