Bias adjustments?

[Gazzip]

Quick question for those in the know about Bias:

I can only turn my pre-amplifier's volume up to level 4 out of 100 before I am approaching a level above talking. If I get up to twenty then we are getting very loud. Thirty and my room can no longer take it and acoustically overloads. So am I missing anything in terms of my pre's abilities by not being able to explore further up the dial? I have read varying posts and forums about this suggesting that I might be. Many suggest "bolt on" in line attenuators from, among others, Rothwell, but I am reluctant to add something that may subtract something if you know what I mean. Are they transparent? Is it worth getting my power amps' bias adjusted? Last but not least am I worrying about nothing? Thanks! G

 

[Deleted member 108165]

I've got -10dB Rothwells on my Pearl Lite amp and I couldn't be more happy. I find them completely transparent and have given me approx +2.5 hours more volume sweep on the loud knob. You could always try the cheaper adjustable attenuators on amazon, (-10, -15, -20dB), and see what level of attenuation fits your requirements, then purchase the appropriate fixed level Rothwell version. Can't help you with the power amp bias as I haven't got one. Finally, yes you may be worrying about nothing.

I looked at the attenuators as a solution for more sweep on the dial, and therefore, better control without loss of fidelity. As far as this is concerned they work a treat.

 

[Electro]

It seem like you have a output / input level mismatch between your Audio reasearch pre and the bryston power amps or between the DAC and pre amp .

Is the output level of the AR pre or chord DAC adjustable ? If not some in line attenuators may help with the problem .

You have an extremely powerful system that is capable of very very high spl's , what size is your listening room ?

 

[nopiano]

If you are in the UK, I would ask the AR importer, Absolute Sounds

http://www.absolutesounds.com/contact_us.php

...and/or the Bryston importer who I presume is PMC

http://pmc-speakers.com/about/contact-pmc

I would speculate that you are unlikely to be missing out on sound quality as such, but may be unable to make fine volume adjustments and may have channel balance flaws (depending on how the volume pots are implemented). Either of these would be a big weakness in an otherwise high quality setup.

 

[Vladimir]

Adjusting the bias or the DC offset won't adjust the gain levels and you are very likely to destroy your amp or speakers or both.

What is the input sensitivity of your preamp and power amp?

 

[Gazzip]

Adjusting the bias or the DC offset won't adjust the gain levels and you are very likely to destroy your amp or speakers or both.

What is the input sensitivity of your preamp and power amp?

I definitely have an output/input signal level issue between pre and powers. When I ordered the Brystons (Domestic Version) I am pretty sure I could have requested the input level control (Pro Version), which would have enabled a 0 to -14dB attenuation adjustment to the input level. Sadly this was not an issue (or an option) I was aware of at the time of purchase so I have no input adjustment fitted.

i just received a pair of 15dB attenuators in the post. £14 for the pair off eBay and they sound every penny of £14 so I am kind of back to square one. On the basis of what they did to the sound (low frequencies practically disappeared) I am not going to tread the Rothwell attenuator path.

Does anybody know if the input level control can be retrofitted to a 7BSST2 by PMC? I know that I can have the power amp adjusted internally but I am loathe to have that done on the basis that it is not a user adjustment and I may have the opposite problem if I swap out the pre for a model with a lower output level.

 

[Vladimir]

I think it might be an impedance missmatch that affects the low bass. I'm looking at these measurements in the AR review by Stereophile.

 

[Gazzip]

I think it might be an impedance missmatch that affects the low bass. I'm looking at these measurements in the AR review by Stereophile.

Do you mean the raising of the Impedence through the introduction of the attenuators is killing the low bass or are you talking in general terms about the setup?

 

[Vladimir]

I think it might be an impedance missmatch that affects the low bass. I'm looking at these measurements in the AR review by Stereophile.

Do you mean the raising of the Impedence through the introduction of the attenuators is killing the low bass or are you talking in general terms about the setup?

Seems to be. The AR is quite sensitive in that region with impedance matching, as per John Atkinson's conclusion in that review, not my own. By adding the attenuator with just few hundreds of ohms impedance, you may have made the frequency response slope by few dB in the bass area. How much ohms is the attenuator specified?

 

[Gazzip]

Hmmm... This is not good. Presumably the input level atenuation that I may/may not be able to have retrofitted to the Brystons by PMC is going to have the same effect?

 

[Vladimir]

Hmmm... This is not good. Presumably the input level atenuation that I may/may not be able to have retrofitted to the Brystons by PMC is going to have the same effect?

IMO... upgrades and tweaks ruin things in the long run. Why you may ask. Well, the moment you mess with the factory condition of any of those two very high quality components, you litteraly made a Frankenstein and ruined a future classic that should be cherished and refurbished to factory specs in the next 40-50 years or more.

I would simply get a professional technician to gain match both components by making an inline attenuator custom just for your current gear. Maybe someone can recommend a shop near you so you drive there and not risk shipping damages. Maybe the whole thing can be arranged over the phone or email and just ship you the attenuator. Why not even email Rothwell for a custom made one simply based on both the AR and Bryston specs.

*unknw*

 

[unsleepable]

I definitely have an output/input signal level issue between pre and powers. When I ordered the Brystons (Domestic Version) I am pretty sure I could have requested the input level control (Pro Version), which would have enabled a 0 to -14dB attenuation adjustment to the input level. Sadly this was not an issue (or an option) I was aware of at the time of purchase so I have no input adjustment fitted.

According to the brochure of your Bryston, there should be a switch at the back to select between 23 or 29dB gain—this is not only for the Pro model. Do you have it, and have you tried it to reduce the gain?

 

[Gazzip]

I definitely have an output/input signal level issue between pre and powers. When I ordered the Brystons (Domestic Version) I am pretty sure I could have requested the input level control (Pro Version), which would have enabled a 0 to -14dB attenuation adjustment to the input level. Sadly this was not an issue (or an option) I was aware of at the time of purchase so I have no input adjustment fitted.

According to the brochure of your Bryston, there should be a switch at the back to select between 23 or 29dB gain—this is not only for the Pro model. Do you have it, and have you tried it to reduce the gain?

Yeah, thanks unsleepable but it has always been on the the reduced dB setting.

 

[Gazzip]

Hmmm... This is not good. Presumably the input level atenuation that I may/may not be able to have retrofitted to the Brystons by PMC is going to have the same effect?

IMO... upgrades and tweaks ruin things in the long run. Why you may ask. Well, the moment you mess with the factory condition of any of those two very high quality components, you litteraly made a Frankenstein and ruined a future classic that should be cherished and refurbished to factory specs in the next 40-50 years or more.

I would simply get a professional technician to gain match both components by making an inline attenuator custom just for your current gear. Maybe someone can recommend a shop near you so you drive there and not risk shipping damages. Maybe the whole thing can be arranged over the phone or email and just ship you the attenuator. Why not even email Rothwell for a custom made one simply based on both the AR and Bryston specs.

*unknw*

Thanks Vlad! That is really good advice and I will explore before entering Mary Shelly territory.

 

[unsleepable]

Hi Gazzip, I just wanted to provide a few more thoughts about your setup.

By all means, you have an impressive kit. But I think that somehow everything about it seems to be laid out to get exactly the problems that you are having.

At the current gain setting in your power amp, its input sensitivity is 4.6V according to the manual (I calculated 4.86V, but whatever). But then, your preamp is capable of producing 15V out of the single-ended output. This means that the usable rotation of the volume pot in the preamp is actually quite little in your situation. If its gain is linear in relation to the position of the pot, at around 30% rotation you will already be feeding the maximum voltage that the power amp can handle, and it will be working at full power—this also depends on other variables, but more or less it's that.

Add to this that your speakers are not exactly on the insensitive side of things at 91dB, and what you get is very loud sound by just touching the pot a little. Less gain would improve matters.

It's a pity that the attenuators didn't work well for you, but I think that the reason may be related to the impedance mismatch issue that Vlad raised. The load impedance of the power amp should be sufficiently high in relation to the output impedance of the preamp to minimise the voltage loss. I remember hearing a couple of times that a difference of a 1,000 order was optimum—so for example, a preamp with 50Ω output impedance would be matched with a power amp that would present 50kΩ load impedance. But it's more common to find pre/power amps combinations in which the load impedance is 300 or 400 times larger than the output impedance.

Now, your preamp has an output impedance of 600Ω, which is actually high for a preamp. Then your power amp presents a load impedance of 10kΩ on the single-ended input (according to the manual; the brochure says 15kΩ), which is on the low side of things (either). Separately these numbers don't mean much, but when you put them together you are looking at a load impedance that is about 17 times larger than the output impedance of your preamp.

Actually, the product page of your preamp specifies a requirement of 20kΩ minimum load impedance—double what your power amp provides. I think it's also interesting to see that the load impedances of Audio Research power amps, with which your preamp was likely designed to be matched, are usually 150kΩ or larger—fifteen times more than your power amp. And I'd bet that Bryston preamps correspondingly have very low output impedances.

As for your question about whether the sound quality of your setup could be affected, I think that that could actually be the case. You already saw with the attenuators the importance of maintaining the relationship between impedances in the preamp and power amp. It could well be that the sound of your system is currently affected by the impedance mismatch, although to a lesser extent than what you heard with the attenuators.

The gain mismatch could also be a problem. Your preamp is working so little that the audio signal is likely close to the background noise of the preamp, and all that together is what the amp is amplifying. Should this be another preamp, the lack of detail might be apparent—but then, maybe background noise is simply not an issue with a preamp of this class, I don't know.

 

[Gazzip]

Hi unsleepable and thanks for the great explanation above. This all leaves me with difficult decisions to make, as I think something is going to have to change in my setup. Do I ditch the pre-amp or the Brystons? I just don't know! The logical solution would be to move the pre-amp on because it will sell very easily, plus changing to a solid state pre would actually suit the way I listen these days which is little and often. I can't leave the pre-amp on due to valves and by the time we are up to speed I usually have to stop. Not always like that but mostly. However I could probably get a reasonable return on the Brystons (5 months old) and put that towards an ARC power. That could sound amazing but more valves=more hassle. Decisions, decisions, decisions.....

 

[unsleepable]

Glad to be of help. Keep us posted about your progress!