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square_wave 29th May 2007 02:42 PM

Measuring DC at preamp output
How do you go about measuring DC at the preamp output ?
Most valve preamps have some DC happening at their output for 30 seconds or so after power-up. Can I measure this with a multimeter ?

anatech 29th May 2007 03:57 PM

Hi square_wave,
Yes, just use a high impedance model. The transient DC is from the capacitors charging. They will reverse polarity when you turn the amp off, but should be a lower amplitude as the rate the caps discharge is much longer than the charging rate.

You should have no DC voltage once the preamp has settled down.

You might find an oscilloscope may give you a better idea what s going on. Use a very low sweep rate.


square_wave 30th May 2007 10:05 AM

What would be a simple oscilloscope model from brands like Tektronix, Philips etc…Something cheap but useful and reliable?

anatech 30th May 2007 01:44 PM

Hi square_wave,
Try the 22xx series from Tektronix, a 2213A or 2235 would be excellent. If you can score a 2235, you'll find many uses for it over the years. It's a very good scope. Look at all those in the series.

Philips ....... I've had reliability problems with them. I wouldn't touch a combi scope, but something less than 100 MHz may be more reliable. Avoid the cursor models. I have a PM3270, great display but it tends to fail. Heat is a big problem.

Some of the newer Leader (and maybe Kenwood) 'scope had vertical modules that tended to fail. Tektronix with their 24xx series also have this problem.

Many of the new, inexpensive analog 'scopes look like they'd be fine. That may be a way to go to. Look for the horizontal sweep rate. 0.5uS / div at the least. Also look carefully at the vertical sensitivity and the amount of noise on the trace. Digital scopes are useless in audio until you reach the 3000 series Tektronix or 3000 series in Agilent. The cheaper 'scopes have cool features, but these are useless if you can't acquire enough detail to work with. There is a thread on this in "Electronics and Parts",


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