Oliver Archut custom made v78 pre for mastering - HPF switch not working?

[Darcy]

Hi all,

Would really appreciate some help here. I have a TAB Funkenwerk v78 preamp that Oliver made custom for a mastering engineer. Uses a different tube in the second stage, looks like it has some special xofrmers etc - whatever it is it sounds great. But the 120Hz HPF isn't working.

I traced the circuit of the LPF section. Also attached is original v72 circuit for reference.

Any reason this isn't working? I thought was faulty cap so replaced but same issue. Switch seems to be working fine too.

As a heads up I am a producer with basic electronic knowledge and super basic tube amplifier understanding. I might be missing something obvious? I don't know how to calculate the RC values so maybe it's doing some kind of 20Hz rolloff since it's for mastering? If I wanted to make it 120 - should I change cap or resistor?

Again any help greatly appreciated - thank you!

EDIT: capacitor is 0.022uf, not 0.22uf.





=

 

[radardoug]

If it is wired as drawn, you have a short across the switch. It will never work.

 

[capacitorless]

Just my .02 and I'm dead tired, but I think the connection between 1 & 2 is to avoid a pop when switching the HPF on and off. When the switch bridges 2 & 3, the .22uf cap is bypassed. Edit: there may well still be an interaction with the input transformer secondary and the cap in this position.

The 210k resistor could be a reference for the grid (eg grid leak?), but unsure of this and how it might interact with the .22uf cap.

My vote would be to increase the value of the .22uf cap. This is what I'd do in a guitar amp to increase the low frequency bandwidth (and vice versa for removing bass from a signal).

Edit: doh! It's been a rough week so far. I meant to say to *reduce* the value of the cap as dmp recommended below, which will make the knee higher than it is currently.

 

[dmp]

The 3dB high pass frequency for 0.22uF & 210k is about 3 Hz. Formula is 1/(2*pi*c*r)
To get to 120 Hz you would use a smaller capacitor. About 6 nF.

 

[Darcy]

Thanks very much everyone. I made a mistake with the 0.22uF, my apologies. Meant to say 0.022uF. Going by what you shared dmp, that's about 30Hz. 30Hz @ 3db rolloff is about what I would expect for a mastering situation.

Capacitorless, I'm pretty sure you're right, Oliver had done that with all the switches on this unit.

Any recommendations for type of cap to replace with? Polyester, polypropylene, metal film? Existing was a 'green chicklet' type. And what voltage does it need to be rated at? Current is 400v, but does the grid see any serious voltages?

Thanks again for your help!

 

[capacitorless]

I think the green chicklet type is probably Mylar (polyester). Unsure why it would be rated that high.

Another cap type to consider might be polystyrene. I'm thinking of loading up my planned preamps and eqs with them, after reading recently about the quality of sound they are supposed to convey.

edit: this thread at audiokarma goes into this a bit, and one poster says that replacing with polypropylene was an improvement.

 

[Darcy]

Fantastic thank you capacitorless, that thread is great. Appreciate your help!

 

[capacitorless]

There's gotta be a thread (or threads) like that here at GDiy, but I'm literally too tired this evening to search further, haha! Hopefully someone with more knowledge will drop in and offer more enlightenment. Despite my moniker, I'm solidly in the early part of the learning curve for a lot of this

In that thread, paart mentioned "Substituting polypropylene, polycarbonate, or polystyrene for polyester capacitors will not always produce an improvement in audio quality. It depends on where and how the cap is used in the circuit."

I would like to know when it is most appropriate to use (say) a polypropylene cap vs a polyester cap, so I can make the same decisions as you're making here.

 

[Darcy]

I wish I could help you out here but I'd imagine someone else is better equipped to answer you question! As you said also there may be some good threads on the matter. It may also be a matter of trial and error?

 

[dmp]

You can use a low or high voltage cap. I usually start with size and config (axial/radial) and then go from there. Its something that you're better off buying a few film types and trying them than getting an 'answer' from the Internet about.
Mallory 150, wima (mkp fkp and others), solen, styrene.

 

[capacitorless]

@Darcy sorry I wasn't asking you specifically, just pointing out I would like that knowledge as well It's on me for posting later than I should.

On the voltage rating topic, my understanding is that usually the grid is at a level of a few volts, and if you double that (whatever it is) that should be the max voltage that the design calls for. You could double that max expected voltage to be safe. The only things I could think of that would change that would be a specific tube failure (ouch) and that would be unlikely.

 

[Darcy]

Thanks for your answers dip and capacitorless. Ordered the appropriate cap and will install!

 

[capacitorless]

If you're going right away to polystyrene, know that they are sensitive to being overheated during installation. The dielectric is the same general stuff as food utensils (and a bunch of other things) and has a melting point of approximately 100C

 

[inkster]

I remember Oliver telling me(as I don't read german) the cap across the primary windings on a V72(and some other models) is brought out to the connector and is the HPF, I believe the cap on the grid would be used to suppress pops...but I'm not sure.
I would check the switch itself.

 

[dmp]

On the voltage rating topic, my understanding is that usually the grid is at a level of a few volts, and if you double that (whatever it is) that should be the max voltage that the design calls for. You could double that max expected voltage to be safe. The only things I could think of that would change that would be a specific tube failure (ouch) and that would be unlikely.

The grid should be VERY close to 0V. Tubes have a very small grid current, but that is pulled to 0V through the 210k resistor. The cathode resistor provides the bias, of a few volts, making the grid to cathode a few volts negative. If the grid is a few volts, the tube isn't biased and the amplifier will have distortion. The AC voltage for +4 dB is 1.228V, if you happened to connect a line level signal. Unless you plan to hook up a power amp to it, basically any film cap of 5 or 10 volts should be enough.

There are some film caps that will generate distortion around the crossover point and higher voltage caps will reduce this (ref Doug Self).

 

[capacitorless]

Man I don't know why I assumed fixed/grid bias, when there was in fact a cathode resistor present. Thanks. I need more coffee.

Is it a general rule to go higher in voltage ratings for better sound quality, disregarding space and cost? I definitely get what you've said about trying them out vs generalizing using internet advice, though.

 

[dmp]

Is it a general rule to go higher in voltage ratings for better sound quality, disregarding space and cost?

First time I saw that was recently for a phono preamp design by Doug Self. He wrote that for the HPF higher voltage film caps will have lower distortion around the crossover. But I don't know what the mechanism is or how generally applicable it is.
And, sometimes you might like caps that generate distortion.

 

[Bonnie1]

Hello Darcy,
If you really want any of Oliver's designs working as new, or just need a replacement part it's best to contact AMI/TAB directly. [email protected]
-dennis M.