DC in slagleformer preamp

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anwar1965

DC in slagleformer preamp
« on: 20 Apr 2016, 04:58 am »
Hi,

I have 2 passive preamp systems built using slagleformer, one of them fully balanced silver with external 3 single-ended channels (copper slagleformers) for home theater.  The other one is all copper slagleformers for single-ended stereo with 3 additional HT channels in the same chassis.  They are used in different systems where HT and 2-channel are combined, and no HT bypass is required.

For some sources, I have DC problem. For example, when changing volume level, I can hear some pops and clicks.  The higher the volume level, the louder the pops and clicks.  I also have 2 autoformer failures which John replaced.  We believe the relay failures are caused by DC.  Both autoformers that failed were directly connected to OPPO 95 center and rear channels (no input switching).  The OPPO 95 has DC-coupled output.

If I add coupling capacitor for each positive signal before the autoformer, how should the uF values be calculated?  I know that at volume level 54 (unity gain), the source will see the impedance of the power amp.  However, at other volume levels, the impedance will be different.  How to safely select the correct uF value so that the high pass filter operates at frequency below 20 Hz at all volume levels?

The power amps I use have impedance values of 10k to 22k.

Any help is highly appreciated as I am not an electrical/electronic engineer.

Thanks,
Anwar

JohnR

Re: DC in slagleformer preamp
« Reply #1 on: 20 Apr 2016, 09:29 am »
I'm sure John will be along to provide an official answer, but the minimum input impedance will be with the autoformer at 0 dB attenuation. So size the capacitor for the amplifier input impedance.

anwar1965

Re: DC in slagleformer preamp
« Reply #2 on: 22 Apr 2016, 01:36 am »
Thanks JohnR.  But the slagleformer also has gain up to +7 dB.  If I want the coupling capacitor to be good at all volume levels from -54 to +7 dB, then the capacitor uF value will be very large indeed since at +7dB the impedance is around 2k.  Example given below for McIntosh MC303 amp which has 10k input impedance.