Amplifier Problem - Need help!

[maxzimum]

Hi,

I use an 845 Tube based power amplifier and just got a brand new SS preamp to replace an ARC.
I am encountering a strange problem. When I take the volume control past 46 on the display I start getting a high pitched whine which becomes a loud hiss with intermittent cracking past 50. At 45 it's dead quiet. This happens regardless of there being an input connected to the preamp or not.

Could this be an impedance mismatch? Any way to diagnose/fix the problem? I've sent a message to both manufacturers and am yet to hear from them.

The specs are below:

PREAMP
Input resistance:
10Kohm RCA
600ohm XLR (This is the input I use)
Output resistance: 75 ohms
Maximum gain: High Gain 12dB
Max output voltage: +/- 20V

POWERAMP
Input sensitivity 1V for 200W output @ 6 Ohms
Input impedance 10Kohm
Output Impedance 4-8 Ohms

Thanks..

 

[mastodon]

It's an oscillation. The first thing to do is to find out if it originates in the preamp or in the power amp. If you connect the new preamp to another power amp, does the problem still occur?

 

[maxzimum]

Hi, I am going to try with another power amp later today to confirm if there's any problem with the preamp.
In case the preamp is OK, what can I do to remedy the problem?

 

[Palustris]

Well, just from the specs you give above it is clear that you don't even need a preamp. Notice that the amp has an input sensitivity of 1 volt. That means that it will be driven to full power output (probably 20W not 200W) by 1 volt. Now look at the specs. for the preamp: maximum output is 20 volts!

It is no wonder that you can't turn up the preamp; you are driving the amp to full output with the preamp idling. You don't need a preamp with that amp. Does it have a level control on the input? If not get a passive preamp.

 

[Palustris]

Just a reminder: standard specification for CD player output voltage is 2 volts. That's why you don't need a preamp with this amplifier.

 

[classicaudio]

Yes, it is an oscilllation, and yes, the output voltage of your preamp is way in excess of what you need. One solution here is to acquire a passive preamp, so that you'll have input-selection capability.

 

[maxzimum]

Hi,
An electronics engineer friend suggested I put a 10K resistor in parallel at the Power amp input which has sorted the problem.

Palustris, I did try my DAC direct to the power amp (volume control from PC) to try and got the same (probably louder) noise as described above with no input on the DAC. I also do need a pre since I have 2 sources and my power amps have no controls.

Thanks

 

[maxzimum]

I jumped the gun. The noise is not there if I select an input on the pre that's not connected. The moment I select an input connected to the source (DAC in this case), the noise starts although at a higher level.
On another note, I am getting a buzz/humm (like a ground loop). Could these be related?

 

[maxzimum]

I have been recommended to add a Line input transformer at the power amp to sort the buzz and also increase the impedance.
Can someone suggest the right model from Jensen/Lundahl based on the specs above?
Thanks

 

[Alkis]

With a passive preamp as suggested, you will overcome all of these problems. And you can add source selector too.In my opinion,this should be the first attempt.
Just remember, that volume control has to be far bellow 10k. If the source is 50R, then a 600R pot would do the job nicely.

 

[maxzimum]

Will try a passive preamp at some point.
At the moment, I would really like to get the preamp to work with the power amp.

 

[amptech]

Try this site cool schematics for some valve/solid state circuits..

 

[maxzimum]

Try this site cool schematics for some valve/solid state circuits..

amptech, can you repost the link to the site, please?

 

[amptech]

I've rechecked on line... it's there, COOL SCHEMATICS

what type of preamp are you working on?

 

[amptech]

http://COOL SCHEMATICS

 

[maxzimum]

Got it thanks..
Its a preamp I bought, not something I'm making.

 

[amptech]

Your pre amp is it faulty? do you have test equipment such has a scope to check signal out of the preamp..

 

[amptech]

most pre-amps will drive power amplifiers..some power amplifiers have in put voltage of say 775mv to 1 volt.

 

[Alkis]

One thing I don't understand is why so much discussion about gain. Vout of a typical linestage is x 2,5. Further more a typical dac or cdp is rated at 2.1V.
So,what should be considered about is the impedance matching.Not the amplification factor.

 

[amptech]

Standard impedance used to be 47k ohms or even as high as 100k on most oldish equipment and I came across amps with impedance as low say 10k so, along as you don't push the amp to clipping... try and apply your pre-amp to your amp and ease the volume up..