diyAudio

diyAudio (http://www.diyaudio.com/forums/)
-   Analog Line Level (http://www.diyaudio.com/forums/analog-line-level/)
-   -   Gain regulation on Counterpoint SA3000 (http://www.diyaudio.com/forums/analog-line-level/242327-gain-regulation-counterpoint-sa3000.html)

sattwa 11th September 2013 11:46 AM

Gain regulation on Counterpoint SA3000
 
Hi, do you know if one can change the output gain in the SA3000 preamp?
On the board I see a trimmer for each channel (see at the bottom of the pic, they are blue with white screws)
Are they used to regulate the gain?
Manythanks :)

http://imageshack.us/a/img547/1953/op9x.jpg

VivaVee 13th September 2013 06:43 AM

They should be marked VR610A and VR610B. They adjust line stage gain for channels A and B, respectively. The adjustment allows you to move from 100% 'normal' gain down to 80% 'normal' gain.

sattwa 13th September 2013 07:48 AM

Hi, thank you for your reply!
I tried yesterday to rotate those pots and they did change the gain. So now my SA3000 is well matched with my Marantz 8B. I changed the level so that I have the maximum volume I would stand when the volume knob is at 12.
However from this regulation I was hopping to have less hiss in the horns of my Cornwalls.

I cannot quite understand.. If I use the SA3000 with my Luxman M300 I can strongly reduce this hiss by changing the power amp sensitivity (by rotating the two pots on the power amp), so I thought this hiss was due to a too high gain from the preamp. But indeed changing this gain does not actually change the noise in the horns..

VivaVee 14th September 2013 01:01 AM

Quote:

Originally Posted by sattwa (Post 3630321)
However from this regulation I was hoping to have less hiss in the horns of my Cornwalls.

I cannot quite understand.. If I use the SA3000 with my Luxman M300 I can strongly reduce this hiss by changing the power amp sensitivity (by rotating the two pots on the power amp), so I thought this hiss was due to a too high gain from the preamp. But indeed changing this gain does not actually change the noise in the horns..

The trimpots do NOT change the gain of the linestage (or its noise contribution). The trimpots are in series with the volume control so adjusting them changes the signal level BEFORE the preamp line stage.

Adjusting the M300 is changing the signal level AFTER the preamp line stage.

sattwa 14th September 2013 08:16 AM

ah, my ignorance.. I thought the volume pot controlled the output, non the input of a preamplifier. Anyway in my ignorance I believed that what matters is the current that is seen from the amplifier. Can you explain this a bit further?

Do you also know if there is anything I could do to reduce the hiss?
Thank you

sgrossklass 14th September 2013 11:52 PM

Quote:

Originally Posted by sattwa (Post 3631794)
Do you also know if there is anything I could do to reduce the hiss?

Tube circuits without any input and output transformers (and I don't spot any here) tend to be on the noisy side of things. In theory it shouldn't be that bad (as tubes can get into single-digit nV/sqrt(Hz) territory), but in practice it often is. Maybe a tube wizard would spot something if they had the schematic, but I wouldn't get my hopes too high. The fellow who designed the unit seems to be on the very, err, subjective side of the hi-fi hobby. Noticeable hiss may actually have been desired, at least its effect of subjectively "softening" the sound is well-known.

VivaVee 15th September 2013 02:08 AM

Quote:

Originally Posted by sattwa (Post 3631794)
ah, my ignorance.. I thought the volume pot controlled the output, non the input of a preamplifier. Anyway in my ignorance I believed that what matters is the current that is seen from the amplifier. Can you explain this a bit further?

Do you also know if there is anything I could do to reduce the hiss?
Thank you

You have not mentioned whether the problem is with the phono or line inputs or both.

Nonetheless, what you are missing is an understanding of the typical gain structure of a typical preamplifier. Don't worry as it is a very common misunderstanding. The phono stage and line stage gains are both fixed. The volume control (and the line stage trim, in this case) can only reduce the signal level going into the line stage, which then increases the signal by a fixed amount. So the volume control appears to reduce the line stage gain but in reality it simply throws some signal away.

The solution is an attenuator (volume control, if you like)) after the preamp line stage which is precisely what you have with the M300. A simple resistive L-pad would do the trick - you could build one into a RCA adapter and plug it into the output of the preamp. This would solve your gain/noise problem without modifying the preamp or power amp.

VivaVee 15th September 2013 02:26 AM

Quote:

Originally Posted by sgrossklass (Post 3632460)
Tube circuits without any input and output transformers (and I don't spot any here) tend to be on the noisy side of things. In theory it shouldn't be that bad (as tubes can get into single-digit nV/sqrt(Hz) territory), but in practice it often is. Maybe a tube wizard would spot something if they had the schematic, but I wouldn't get my hopes too high

Valve circuits tend to be noisy because the gain is typically too high for modern line level sources which typically do not require any gain at all. Note the string of 'typical' qualifiers - a valve buffer circuit would provide the desired impedance transformation with low nose while satisfying the desire to use valves.

Quote:

Originally Posted by sgrossklass (Post 3632460)
The fellow who designed the unit seems to be on the very, err, subjective side of the hi-fi hobby

Yes, he actually listened to his designs. Scary idea.

Quote:

Originally Posted by sgrossklass (Post 3632460)
Noticeable hiss may actually have been desired, at least its effect of subjectively "softening" the sound is well-known.

The first statement is untrue in this case and the second is a gross generalization. There are many different noise sources - some may mask unpleasantness but the majority are simply unpleasant by their very presence.

In the end, noise is a system issue. The horn-loaded Cornwall speakers are likely to require less gain than a more typical loudspeaker system.

sattwa 15th September 2013 09:07 PM

Quote:

Originally Posted by VivaVee (Post 3632540)
You have not mentioned whether the problem is with the phono or line inputs or both.

Nonetheless, what you are missing is an understanding of the typical gain structure of a typical preamplifier. Don't worry as it is a very common misunderstanding. The phono stage and line stage gains are both fixed. The volume control (and the line stage trim, in this case) can only reduce the signal level going into the line stage, which then increases the signal by a fixed amount. So the volume control appears to reduce the line stage gain but in reality it simply throws some signal away.

The solution is an attenuator (volume control, if you like)) after the preamp line stage which is precisely what you have with the M300. A simple resistive L-pad would do the trick - you could build one into a RCA adapter and plug it into the output of the preamp. This would solve your gain/noise problem without modifying the preamp or power amp.

Hi, thank you again for your help.
The hiss is from the line stage (therefore also from the phono stage). I should say however that this is not a big issue. As you said part of it is surelly due to my speakers unforgiveness (being 105 dB).
Anyway it is not impossible to be quite even on my Cornwalls, I have tried other valve and ss amplifications who were very silent.
The SA3000 sounds very good, and it is quite amazing for what it can give at its price.
However I was expecting something more with respect to noise level.
I can use attenuators at the outputs but I guess this would not give much honour to Mr Elliot..;)

VivaVee 15th September 2013 09:13 PM

Quote:

Originally Posted by sattwa (Post 3633491)
I can use attenuators at the outputs but I guess this would not give much honour to Mr Elliot..;)

Just think of them as attenuators on the input of your power amplifier and I am sure he would forgive you :D


All times are GMT. The time now is 12:06 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio


Content Relevant URLs by vBSEO 3.3.2