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Old 17th December 2015, 12:48 PM   #1
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Default Quicksilver full function preamplifier

Hi!

I have a Quicksilver full function preamplifier in daily use. But when power on, the amp swing approx 60V on the outputs and that kill my DC-coupled transistor poweramp - if I don't remember to push in the "mute" switch.

I there any mods to fix that?
A relay with 30 sec delay on and 0 sec off - perhaps?

Any suggestions?
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Old 17th December 2015, 01:45 PM   #2
DF96 is offline DF96  England
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As it appears to claim a -3dB LF rolloff at 0.1Hz it probably has output coupling capacitors which are 20-50 times too large in value. This will not help!
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Old 17th December 2015, 02:52 PM   #3
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Put a resistor across pre amp output to discharge any DC.
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Old 17th December 2015, 03:11 PM   #4
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It is a slow swing ac topping 60v when I turn amp on. Gets into 0v after 30 sek. In that period I must have pushed in "mute". If I forget to do so, my poweramp fry! I think the mute button shorting output to ground.

I need to do some change to prevent amp IF i dont remember to mute.

Resistor will not do the job.
Change components in amp - I dont want to; Mike Sanders at Quicksilver do have a reason for have a big cap - i hope....?
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Old 17th December 2015, 04:03 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Gladiator33 View Post
<snip>

Resistor will not do the job.
Change components in amp - I dont want to; Mike Sanders at Quicksilver do have a reason for have a big cap - i hope....?
Not necessarily a rational one, and it's certainly not a very good idea if the load is a solid state power amp. (It's not really a great idea to do that to the input of a tube amplifier either.)

A -3dB point in the 1 - 5Hz range is certainly a better choice, I can't imagine the size of film cap required to get that LF corner, but it must be huge which likely means plenty of parasitics and less than ideal performance.

You could use a 555 timer IC to drive a relay and unmute the pre-amplifier after 60 seconds or so. Should be designed to mute immediately upon removal of power and fully reset for the full mute time.
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Old 17th December 2015, 04:53 PM   #6
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I put a thermistor in series with the transformer lead to the power supply. This Pre also kills power supply tubes and this helps with that also.
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Old 17th December 2015, 05:02 PM   #7
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A 555 circuit is over the top for this application. The linked to circuit has never let me down and can be built from easily accessable parts bin finds. Though the circuit is described as a speaker delay circuit, it can easily be applied to the line output of a preamp.

Simple delay speakers &ndash; Electronic projects circuits

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Old 17th December 2015, 05:04 PM   #8
rayma is online now rayma  United States
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Quote:
Originally Posted by kevinkr View Post
You could use a 555 timer IC to drive a relay and unmute the pre-amplifier after 60 seconds or so.
Should be designed to mute immediately upon removal of power and fully reset for the full mute time.
Yes, in a tube preamp, connect a discharge diode between the timing capacitor and the timer supply,
which should be the DC filament supply (it must be ground referenced). This voltage will collapse quickly.
I've been building a similar circuit for many years. Use a low leakage electrolytic for the timing function.

Last edited by rayma; 17th December 2015 at 05:11 PM.
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Old 18th December 2015, 09:18 AM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by Gladiator33
Mike Sanders at Quicksilver do have a reason for have a big cap - i hope....?
Yes, I'm sure he had a good reason. Selling product to people who think that a 0.1Hz rolloff is 10 times better than 1Hz may be his reason.
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Old 18th December 2015, 10:14 AM   #10
Merlinb is offline Merlinb  United Kingdom
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Put a pair of 16V back-to-back Zener diodes across the output connector.
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