New user from France ! Hello

[ytsexav]

Hello to all of you !

I was given this adress by a pro who said someone could help me there.
Here is my problem :

I have a copy of Marantz 7 built in China, very good preamp thoug, but works strange with my system.
Before using it, i had a passive preamp, just a selector and an ALPS volume pot.
It was plugged between a Pioneer PD75 ELITE Cd player and a pair of AESD active bi amplified in class A monitors.
Everything was perfect, but i wanted to go to valves...

I received and plugged my new preamp, after changing the chinese valves (good by the way...) to Sovtek 12AX7LPS and Valvo ECC82.
Sound is amazing but it seems i have too much gain now...
Whatever the volume output, the sound is distorted in the preamp.
To fix the problem before finding a better solution, i put the passive preamp between the source and the preamp to lower the input volume.
Now it works perfectly, but i know this is not a good solution.

What should i do ???

I bought a pair of 5751's valves to lower the preamp gain, but still waiting to receive them.
Maybe i should tweak the preamp and add a very good quality resistance made reducer after the input's RCA (i shunted the selector, which i don't use anymore).

Don't know what to do, but happy to have problems, cause it'll improve my knowledges in valves

If anyone can help...

Thank you very much

Xavier from France

[pre65]

Sound is amazing but it seems i have too much gain now...
Whatever the volume output, the sound is distorted in the preamp.
To fix the problem before finding a better solution, i put the passive preamp between the source and the preamp to lower the input volume.
Now it works perfectly, but i know this is not a good solution.

What should i do ???

If anyone can help...

Thank you very much

Xavier from France

Hi Xavier-one of my amps suffers from a similar sensitivity problem and i use the same solution as you,as i had one to hand.

It does the job !

You could try converting the preamp volume control with a "shunt" mod.

[simon]

Hi Xavier.

I was given this adress by a pro who said someone could help me there.

That "pro" is me, and as my friends here will testify I'm really not a pro

I don't know the circuit but Xavier thinks the pot is after the gain stage, not before, so a quick fix might be to replicate the passive pre with a divider placed between the input selector and grid of the first valve.

But then I wondered if there was something else going on with mismatched sensitivities or impedances, and soon got out of my depth.

Anyone know the circuit or have any thoughts?

[Nick]

Looking at the circuit, the best bet might be as suggested convertuing to useing a shunt volumn control. Or adding a attenuator beteeen the volumn/bal and the first valve.

[simon]

Xabi,

If you wanted to add an attenuator you could do something like this:

Your passive Alps pot probably looks something like the first attachment. If you consider the pot as two resistors - one above the set volume and one below - you can replicate this inside your pre something like the second attachment.

I've drawn them as 40k and 60k to illustrate but they could be anything within reason. Perhaps the best thing to do is to set your passive to the volume you like best and then measure the value of resistance of the top part of the pot and then the bottom. So it could be 30k/70k or 50k/50k etc.

Choice of resistor is personal. It could make a difference but remember that there are lots more resistors in the signal path. You may prefer a metal film or a carbon composite or something else, but they'd have to be pretty poor resistors to be worse than the Alps pot you're using at the moment.

Alternatively you could try the shunted pot on the FAQ elsewhere, and those that have tried it have liked metal films with Alps.

Hope this helps,
Simon.

[Greg]

You're being teased a bit! This is the link you want, but it would be good for you to hunt around on the site for linked information because there has be alot of discussion on this on this forum.

http://wduk.worldomain.net/forum/showthread.php?t=198

[simon]

Er, why is Xabi being teased Greg?

[Nick]

Not sure about the teasing either, but anyway, what you could do is look at this bit of the circuit above I have attached the before and after, I would try maybe a pair of 47 or 100k resistors to reduce the volumn.

[Greg]

Hi Simon,

No offense intended. You mentioned the shunt mod on an FAQ elsewhere. Being new here, Xabi might find that a bit of a tease to explore. I just filled in the gap

Best wishes,

Greg

[simon]

Hi Greg,

No offence taken . I'd already given Xabi that link by email so didn't bother posting it again. Perhaps I should have done for completeness.

Simon.

[ytsexav]

Thanks to Simon and all of you !

I'll soon try the shunt volume pot to see if it works ! Anyway, learning a lot which is great.

If anyone have other suggestions, i'm still listening and learning.

Kind regards

Xabi