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Old 25th March 2007, 01:33 AM   #1
HOEK is offline HOEK  Netherlands
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Default passive preamp

Hello

I wil make a volumepot in my poweramp
Ouputlevel cd-player is 2V at 50K
Impedance poweramp is +/- 40K
What is the most suiteble value for the pot 50 of 100K?




Thanks
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Old 25th March 2007, 02:07 AM   #2
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Check the output impedance of your cd palyer again. A 50k Zout sounds too high. Or do they mean thet recommend a 50K load?.
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Old 25th March 2007, 10:56 AM   #3
HOEK is offline HOEK  Netherlands
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Hello

The loadimpedance must be over the 10K
I guess 50k is the max Outputimpedance off the cd player,
those will than give 2V rms
I wonder what value pot match the poweramp (imp. 40-50K)




regards Hoek
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Old 26th March 2007, 07:46 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
if the source impedance of the CD player is below 1k0, then that fits with the recommendation for a load exceeding 10k.
the power amp will also appreciate a source impedance below 1k0 and even better if below 200r.

A passive pre-amp cannot meet all these conditions.

The best you can hope for is 10k volume pot presenting a 2k5 source to the power amp. This arrangement will desparately need very short cables and even then the filters built into the power amp input will act as variable tone controls as you adjust the volume.

Passives can sound nice but they have their downside.

To answer the power amp source question.
If you wanted to present a 200r impedance to the power amp then you would use a 800r volume pot. That would need a source player that could drive a 600ohm load to 2Vac (3Vpk) from 10r source impedance.
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regards Andrew T.
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Old 26th March 2007, 01:28 PM   #5
Gordy is offline Gordy  United Kingdom
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HOEK your CD player output impedance is not 50k. It will be much, much lower than that, typically 50 Ohms to 2k Ohms.

Maybe they recommend a minimum load of 50k? Using a different load will not cause physical damage.

Anyway, between a typical CD player and an amp with a 40k input impedance, the best compromise for a passive volume control would be a 10k potentiometer. As AndrewT said: keep your cables short (...and possibly try and find some with low capacitance).
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Old 27th March 2007, 08:29 PM   #6
HOEK is offline HOEK  Netherlands
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hello

My source is a sacd-player type scd-555es
ouputlevel 2V rms (at 50Kohm)
Inputimpedance poweramp is 40-50Kohm
Actually it is playing with a log.pot 10K
The sound is very good
lenght signalkable potmeter-input poweramp is circa 60 cm
Is it usefull to put the volumecontrol in the amp?
Will this better the performance?
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Old 28th March 2007, 07:20 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
what filters are built into the power amp input?
What filters are built into the source output?
What is the capacitance (pF/m) of the connecting cable?

Give us the info and we can assess what the 10k pot and long cables are doing to your frequency response.
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Old 28th March 2007, 07:01 PM   #8
HOEK is offline HOEK  Netherlands
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hello

My amp is a JLH 69 (original)
Capacitance conexioncable and outputfilter in cd-player are
unknown.


regards HOEK
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