V78 Frequency Response

Hi All,

I have a couple of Siemens V78 on the bench that are in the original spec and have not, as far as I can see, been modified in any way.  They seem to work fine in giving the 40 dB to 70db gain, but the frequency response has a couple of nasty peaks: one at about 5Hz of 5-6dB and another at about 39kHz of about 9 dB.  Otherwise they are flat 20Hz to 20kHz

Are these peaks normal for these beasties?
If not what could be the cause and how do I fix them?
Are they of concern, being outside of the audio spectrum?

I have also read that V78s can be converted to V72s.  Is this worth it and who might be able to do it.

Thanks for any advice

Mike

The 5 Hz is probably the resonance of the output capacitor with the inductance of the output transformer.
(But how often do you record 5 Hz or 39 KHz?...)

With the output properly terminated the resonance should be much lower. Anyway, the first use for these was FM broadcast which has been filtered at 50Hz and 15kHz in Germany anyway. So the troublesome frequencies are out of the way.

I used to own a pair of V78's. Oliver Archut told me the correct termination is 700 ohms if still configured as original on output transformer.

I'd terminate the beast properly as mentioned, then there is a trim cap inside which should help to adjust the 38kHz peak. Keep in mind that the V78 was not considered a proper micamp but it was actually a talkback amplifier, so it might be more bumpy. The frequency response is supposed to be +/- 1dB from 40 Hz to 15kHz with a steady decline to 200 kHz. Anyway they sound awesome, I wouldn't worry too much about these bumps so far outside the hearing range. Having adjustable gain which is quite useful obviously I wouldn't consider reconverting them to V72 which is fixed gain by nature. Obviously that depends on what you want to use it for.

Michael

Hi guys,

Thanks for the replies and advice. 

The response is the same when terminated by 680 ohms,

Yes my thoughts were that the peaks were outside the 'worry zone' so leave as is and enjoy.  Athough I have to admit, I could not find the 40pF trimmer anywhere - any wise words?.

Kind regards

Mike

madswitcher said:

Athough I have to admit, I could not find the 40pF trimmer anywhere - any wise words?

Click to expand...

If it is like the feedback loop in the V77 then it is somewhat of a one-way trimmer. The component looks more like a wire-wound resistor but actually what is going on is that there is an electrode in the middle connected to one end with an insulating sleeve around it and the other end connects to a tight coil which winds around the sleeve and is open circuit at the far end. The whole lot is covered in a paint, so the coil is adhered and peels away at the open end quite neatly.

I assume you peel away down to (just above?) the capacitance level you want and then trim the excess wire off. It does mean if you go too far or want to increase the capacitance you need a new component, or I guess you could stick something of similar size in parallel to bump it up again a bit.

Ahhhhhh.  I see - yes I found those, they look as if they are covered in 1940s heat shrink sleeving .   

I think I will leave well alone in this case.

Thanks Matt

Mike

You could just replace it with a normal trim cap and see what you can achieve with it.

Michael