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Old 3rd March 2003, 06:37 PM   #11
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Are we trying to interpret the circuit operation in a different way to the designer? While it appears an inefficient method, is this pot actually there to control the level of feedback? At full on, there is max feedback. Perhaps on different inputs the user is supposed to alter the control to suit personal taste. Turning the control back will not be particularly effective at reducing volume as the reduction in feedback increases amplification. After all, vol and tone controls could appear in a pre-amp which is used with the amp. If you place the vol control in the main amp then why not your bass/treble controls also?

Perhaps this is a cunning amp designed by inscruitable designers to fool decadent western capitalist audio fans?
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Old 3rd March 2003, 06:38 PM   #12
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Or maybe the W of war as the feedback is fighting back!!
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Old 3rd March 2003, 06:42 PM   #13
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Hi Bournville!!

Se also:

http://www.audioasylum.com/forums/tu...ges/26132.html

The pot is efectively a volume pot!



regards

Jorge Santos
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Old 3rd March 2003, 06:53 PM   #14
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Yes, it may act as a volume control, but it may not have been designed for that function, but to control the level of feedback.

We've agreed that putting a vol control in a feedback loop is bad practice, so looking at the circuit it appears wrong. However, the designer may have wanted to give the listener some degree of control over the level of feedback (for whatever reason) so the pot was placed there for that purpose.

It's not what we would do, but who knows what was in the head of the designer? Incidentaly, it's not the first time I've seen a vol control within a feedback loop!
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Old 3rd March 2003, 07:04 PM   #15
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Default The saga of the pot!

I think that if the intention of the designer is to change the level of the feedback it will must have a resistor betwen the ground and the terminal of the pot that is now conected to ground.

For when the pot is in the minimum position the amp don't shut of!

I'm not surprised that you a i also see pots inside feedback loops
...afterall there are a lot of bad designes out there

Regards

Jorge Santos
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Old 3rd March 2003, 07:14 PM   #16
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One thing that worries about this amp is the way the EM80 'magic eye' indicator is connected - if it was meant to indicate volume levels surely it would be after this pot?

Alternatively, if the pot is a feedback control of some kind and the EM80 indicated the level of feedback (minimum display being the maximum feedback) then it would work in this way... (I admit I'm not an expert on 'magic eyes' - but why have one in an amp anyway?)

An alternative explanation to the whole problem might be a slight copying mistake - if the pot had a small cap in its connection to ground it would function as a tone control, which although regarded by many as bad practice is much more common to find in feedback loops. Then the EM80 simply indicates overall volume levels.

I would like to know how old this design is - EL41s are not widely used these days in preference to EL84s.
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Old 3rd March 2003, 08:08 PM   #17
Foes is offline Foes  Belgium
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Default My damned amp

Hi all,

I'm the owner af the amplifier.
THanks to Jorge to share my problem with you all
Believe it or not, the volume amplifier works.
The EM80 works too, and indicate the input level. Infortunately, the input sensitivity is to high and the amplifier tends so saturate for every volume position... If someone can help me to calculate R4 (and R2) to lower the sensitivity, be welcome ! Also if it is possible de lower the feedback, you can explain...
The shematics shows EL41, in fact this amplifier exist also in EL90 (my amplifier) and in EL84 version. The design seems to be from 1998 !?
Rgrds,

Gauthier
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Old 3rd March 2003, 08:35 PM   #18
Foes is offline Foes  Belgium
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Default My damned amp

Hi all,

I am the owner of this damned amplifier.
Thanks to Jorges to have exposed my problem.
First of all, believe it of not, it works not so bad. Volume works (Yes !!) and the EM80 indicate the input level...
The design is from 1998. The amplifier exist in 3 ouput tube version : EL41-EL90 (my amplifier) and EL84. Also, the quality of the components is not so high... (chinese tubes and carbo resistors...)

Well, input sentitivity is to high, but i guess it's possible to lower it by playing with R4 (and R2 for correct bias ?). How to calculate it ? How to lower feedback too (i know, it will raise global gain but if first stage gain is lower ...) Any Idea ??

Regards,

Gauthier
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Old 3rd March 2003, 10:08 PM   #19
EC8010 is offline EC8010  United Kingdom
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Default Have a Mars bar, Bournville - you're right

We're looking at this the wrong way. We see a push-pull output stage with loudspeaker impedances written next to the transformer, assume that it's a loudspeaker amplifier, and chortle at the poor (Hi-Fi) design. But what if it started life as a musician's effects unit? The EM81 would indicate how much compression/overdrive was being applied - allowing repeatable effects...
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The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
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Old 3rd March 2003, 10:53 PM   #20
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Default Nice to have you here with us

Hi foes!

I belive that the first step is to put the pot at the imput...after that i'm sure that the problem of overload of the imput will desapear..

After changing the pot for to lower the feedback raise the value of R14

Cheers

Jorge Santos

Ps. Sincerly wellcome!
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