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output of valve preamp showing a high voltage when turned on
output of valve preamp showing a high voltage when turned on
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Old 30th June 2020, 02:59 PM   #51
baudouin0 is offline baudouin0
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If its an issue connect two small 12v zeners in series across the output they will clip the voltage to +/- 12.7V. Oh sorry already mentioned!
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Old 30th June 2020, 03:01 PM   #52
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by Jviz4u View Post
Thanks to everyone responding so quickly.
The issue is persistent all the time during the usage and not only in the first minutes.

What will be suggested approach to troubleshoot this ?
Sounds like cap leakage. You need to use a plastic capacitor, or use two electrolytics in series like this (C7, C8, R16, R17, D3, D4). The second cap (C8) can be a low-voltage type. e.g. 25V.
Click the image to open in full size.
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Old 30th June 2020, 05:15 PM   #53
Jviz4u is offline Jviz4u
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What value of resistors and caps you would suggest?
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Old 30th June 2020, 05:27 PM   #54
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by petertub View Post
We are discussing tube amps here, and they typicall has input impedanced in

the hundreds of kohm.
.........................
If one should drive an sandamp with 10k input impedance then something

is needed yes. But how common is that among this groups readers ?
Sorry but no. If you had read the full thread or at least the 4 or 5 posts above yours you would have learned that:

Quote:
Parasound HCA-750A and find out that it triggers Parasound "self-protect"
has as much "sand" as you would find in an average beach
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Old 30th June 2020, 05:47 PM   #55
stocktrader200 is offline stocktrader200  Canada
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0.47uf into 100k gives frequency response rolloff of .5 db at 20hz
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Old 30th June 2020, 06:20 PM   #56
petertub is offline petertub  Sweden
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output of valve preamp showing a high voltage when turned on
Quote:
Originally Posted by stocktrader200 View Post
0.47uf into 100k gives frequency response rolloff of .5 db at 20hz
Seems to me as a reasonable value.
Most speakers don't sound at 20hz, and 0.5dB level shift is not detectable by human.


Using a smaller filmcap would totally eliminate the leakage of an electrolytic
cap, and some say that the sound of eletrolytic is bad.
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Old 30th June 2020, 07:47 PM   #57
kodabmx is offline kodabmx  Canada
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output of valve preamp showing a high voltage when turned on
Agreed it sounds like leakage...

FWIW, I use these as the output of my preamp to drive the power amps, one of which is 5k.

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Old 30th June 2020, 08:42 PM   #58
rongon is offline rongon  United States
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output of valve preamp showing a high voltage when turned on
What do you suppose the input impedance/resistance of a Parasound HCA-750A is?
Probably either 47k or maybe as low as 10k.
That will always be the load for this preamp, so when choosing the value of output coupling cap, that will need to be kept in mind.

The pulldown resistor on the output of the preamp and the input load resistor (or volume control) in the power amp will be in parallel, so even if the preamp pulldown resistor is 1M or something large like that, the power amp's input resistor will still set the load.

If Rload = 10k ohms and
If C = 0.47uF then
f3 = 34Hz

You'll want to make f3 about 5Hz at most, right? For f3 = 5Hz with a 10k ohm load, you'll need a 3uF output capacitor.

A 3.3uF 400V poly-whatever cap should be OK, right? They exist. Or parallel two 2.2uF 400V per channel. That would be close enough.

The MerlinB opposing zener idea looks like it should help here too.

Clamp that sucker!
--

PS - Just noticed that the preamp's B+ is at least +260V, and that output blocking cap is rated at 250V. That ain't good. The output cap should be rated 300V minimum.

Last edited by rongon; 30th June 2020 at 08:51 PM.
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Old 30th June 2020, 08:51 PM   #59
petertub is offline petertub  Sweden
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output of valve preamp showing a high voltage when turned on
Quote:
Originally Posted by rongon View Post
[/FONT]What do you suppose the input impedance/resistance of a Parasound HCA-750A is?
Probably either 47k or maybe as low as 10k.
That will always be the load for this preamp, so when choosing the value of output coupling cap, that will need to be kept in mind.
Code:


---- C1 -----|-----------|
                  |               |
                  R1            Rload
                  |               |
                  |               |
___________|__________|
                  |
                GND
R1 and Rload will be in parallel, so even if R1 is 1M or something large like that, Rload will still set the load.

If Rload = 10k ohms and
If C = 0.47uF then

f3 = 34Hz

You'll want to make f3 about 5Hz at most, right? For f3 = 5Hz with a 10k ohm load, you'll need a 3uF output capacitor.

A 3.3uF 400V poly-whatever cap should be OK, right? They exist. Or parallel two 2.2uF 400V per channel. That would be close enough.

The MerlinB opposing zener idea looks like it should help here too.

Clamp that sucker!
--
I think ( that is my personal opinion) that 20hz is a good lower frequency.
This prevents subsonical noice from intermodulation the bass.
Close to none meda has 20 hz or lower recorded, no musical information
is lost by cutting at 20hz ( cutting at 35hz would only affect maybe 10%

of all recordings )

1uF film cap could be right in this case, cut at 16hz. 0.68uF will cut at 23hz
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Last edited by petertub; 30th June 2020 at 08:53 PM. Reason: Spelling
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Old 30th June 2020, 09:09 PM   #60
Jviz4u is offline Jviz4u
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Quote:
Originally Posted by rongon View Post
What do you suppose the input impedance/resistance of a Parasound HCA-750A is?
Probably either 47k or maybe as low as 10k.
That will always be the load for this preamp, so when choosing the value of output coupling cap, that will need to be kept in mind.

The pulldown resistor on the output of the preamp and the input load resistor (or volume control) in the power amp will be in parallel, so even if the preamp pulldown resistor is 1M or something large like that, the power amp's input resistor will still set the load.

If Rload = 10k ohms and
If C = 0.47uF then
f3 = 34Hz

You'll want to make f3 about 5Hz at most, right? For f3 = 5Hz with a 10k ohm load, you'll need a 3uF output capacitor.

A 3.3uF 400V poly-whatever cap should be OK, right? They exist. Or parallel two 2.2uF 400V per channel. That would be close enough.

The MerlinB opposing zener idea looks like it should help here too.

Clamp that sucker!
--

PS - Just noticed that the preamp's B+ is at least +260V, and that output blocking cap is rated at 250V. That ain't good. The output cap should be rated 300V minimum.

PECIFICATIONS

Continuous Power Output - Stereo:
75 watts RMS x 2, 20 Hz - 20 kHz, 8 Ω, both channels driven;
125 watts RMS x 2, 20 Hz - 20 kHz, 4 Ω, both channels driven
Continuous Power Output - Mono: 250 watts RMS, 20 Hz - 20 kHz, 8 Ω
Current Capacity: 20 amperes peak per channel
Slew Rate: > 130 V/盜econd
Power Bandwith: 5 Hz - 100 kHz, +0/-3 dB at 1 watt
Total Harmonic Distortion: < 0.06 % at full power; < 0.03 % typical levels
IM Distortion: < 0.04 %
TIM: unmeasurable
Dynamic Headroom: > 1.5 dB
Interchannel Crosstalk:
> 80 dB at 1 kHz;
> 63 dB at 20 kHz
Input Sensitivity:
0.775V for full output
Input Impedance: 33 k Ω
S/N Ratio:
> 110 dB, full power, input shorted, IHF A-weighted
Damping Factor: > 800 at 20 Hz
Power Consumption: 240 watts
 

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