How much DC offset can a tube preamp take? - diyAudio
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Old 3rd December 2006, 09:31 PM   #1
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Default How much DC offset can a tube preamp take?

Hi,

I have got this CD player with a pair of 100uF DC blocking caps at the output. I have just ordered some 2.2uF ICW SA caps to replace them. They are big and it will be a bit of effort fitting them in.

Then I just thought why couldn't I just shunt them?

So how much DC offset can a tube preamp take? My tube preamp has a series 100k resistor at the input to reduce the gain. Then there is an ordinary (fake Alps) 50k volume pot after which it goes into the grid of the tube.

I haven't measured the DC offset at the CD player output yet, but presume it is 0.1 Volt, for example. Ignoring the 100k resistor (in another tube preamp I don't use it), 0.1 x 0.1 / 50000 = 0.0000002 Watt dissipation at the volume pot, which seems to be nothing. Even if the DC offset is 1 Volt the dissipation will only be 0.00002 Watt.

Surely any tubes will take 1 Volt DC! Or won't they?

So why are we bothered with DC blocking caps at the output of a CD player?

Does the DC offset problem affect an opamp preamp?

What is the maximum DC offset you feel comfortable to remove the DC blocking caps?

Thanks.

Regards,
Bill
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Old 3rd December 2006, 11:15 PM   #2
DougL is offline DougL  United States
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Quote:
Surely any tubes will take 1 Volt DC! Or won't they?
Tubes are voltage amplifiers. The dc offset would be amplified by mu (more properly Av of the circuit) A 1 volt DC bias on the input could result in a hundred volt change in the output on that tube.
Thinking of it another way, any dc changes the bias of the tube.
This could result in gross distortion, rapid tube aging, or outright tube failure, due to the new bias point.

Regards;

Doug
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Old 4th December 2006, 12:48 AM   #3
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Doug,

Thanks.

I realised it will be amplified but thought, say, under 100mA it would not matter too much, wound it?

Regards,
Bill
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Old 4th December 2006, 01:09 AM   #4
Minion is offline Minion  Canada
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One Problem I just had was I had DC in the Input and it was being amplified so much that my speaker started to Smoke (Speakers don"t like DC at all)...

So even if the Circuitry can take the DC your speaker will most likely not take it and that can be a Very expensive .....

You also won"t be able to replace the 100uF with the 10uF because the Value of the Capacitor allong with the Impedance of the next piece of equipment form a filter and with a 10uF cap you will be filtering out a Lot of the Audio frequencies so you should use the same Value Cap or higher to maintain the frequency responce.....

You should maybe use a High quality Electro (Black Gate??) on the Output which will block the DC and improove the overall sound....

Good luck
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Old 4th December 2006, 01:35 AM   #5
DougL is offline DougL  United States
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Quote:
I realised it will be amplified but thought, say, under 100mA it would not matter too much, wound it?
I would not do it myself. Thatís just Me. I could think of circuits where a volt would not bother it, and circuits where 100 mV might be a deal killer. Without knowing how much dc offset you had, how stable that offset was, and a Pre-amp schematic, itís a no starter IMHO.
I just want you to know the risks, and make an informed decision.
As the DC offset decreases, obviously the less affect it would have. Where the number is good enough is another matter. That is for you to decide.
By the way, your input is a voltage divider. As you very your volume, the bias will change. Also, I do not know how much current a 50k Alps pot will take.

Hope this helps;

Doug
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Old 4th December 2006, 04:17 AM   #6
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Doug,

Thank you again. Your advice is very much appreciated.

I have just built the Aikido with JB's original circuit. No hums. No noise. Running perfectly. There are things I still need to do and am waiting for the part arrival possibly in 3 weeks, before I would say I have completed building it.

Another preamp I built is the simple 12B4A (mu = 6) with a gain of only 3.6 or something (can't remember it on top of my head) without cathode bypass.

I will do the CD player mod sometime this week at which time will measure the DC offset. I don't know the figures.

So basically I was asking a question how much DC offset people would feel comfortable removing the DC blocking caps from the CD player, which will have a dramatical sonic improvement no matter how good the caps are.

You are right and I have not even thought of that - changing the volume pot will change the bias. In some preamp circuits there is an input cap after the pot in which case the bias can be fixed. In some other preamp circuits a volume pot is at the input then to the grid of the preamp, like the two preamps I built. Does it mean the two preamps I have now all have changing bias when the volume is changed? I will give it some thoughts.


Minion,

Tube amps usually have output caps or are transformer coupled, unlike solid state. That is by default. So no DC will be going to the speakers from the tube amps. They use 100uF electrolytic caps at the output of CD players because they want to save space and 100uF would work much than smaller caps. My tube preamps all have at least 100k input impedance so a 2.2uF cap will have f = 1 / (2 * pi * R * C) = 1.000 / (2 * 3.1415926 * 2.2 * 0.000001 * 100000) = 0.7 Hz. I don't even mind the -3dB point at 7Hz (but this is the highest I would go) so a 2.2uF should even work with a 10k impedance, and of course 100k. I don't think any preamps, solid state or tube, would have an impedance lower than 10k. So this is not a problem.

Best regards,
Bill
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Old 4th December 2006, 04:51 AM   #7
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Quote:
Originally posted by HiFiNutNut
My tube preamps all have at least 100k input impedance so a 2.2uF cap will have f = 1 / (2 * pi * R * C) = 1.000 / (2 * 3.1415926 * 2.2 * 0.000001 * 100000) = 0.7 Hz.
Not that it matter much here, but keep in mind that there are probably 47K resistor from out to ground on the output of the cd player which are in parallel with the input impedance, so 100K is probably too high in your calculation.

Anyhow, i think it depends on the tube used. My guess is that the higher the mu, the more of a difference it makes.

Also, sometimes DC on the pot can cause some noise when you turn it. Last, PEC pots are 2W and sound great and are cheap from Digikey if you need the dissipation ability.
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