A Winter's Project

[HoopsOnToast]

Serge, with your experience with the A500, would you think there is any benefit on bridging them? If you did not need the extra power, and the speaker was a relatively easy 8Ohm load would there be any gains?
I am thinking on re-visiting the A500, to use with the AE2's, and having them with a pre this time, as before, I thought it sounded awful when using the volume controls.

[SergeAuckland]

Serge, with your experience with the A500, would you think there is any benefit on bridging them? If you did not need the extra power, and the speaker was a relatively easy 8Ohm load would there be any gains?
I am thinking on re-visiting the A500, to use with the AE2's, and having them with a pre this time, as before, I thought it sounded awful when using the volume controls.

If you don't need the extra power then there's no benefit to bridging. When bridged, each half-amplifier sees half the load impedance, so it gives the amplifier a slightly harder time. I know you said that your 'speakers are an easy 8 ohm load, and the A500 is happy driving down to 2 ohms but nevertheless, if there's no need for the extra power, then I wouldn't bridge.

S.

[griffy]

Think I might try an A500 with my ATC's!

[HoopsOnToast]

Cheers Serge, one A500 will be enough then

[AlmaAtaKZ]

Excellent thread and excellent project. Heard the system at Scalford - best sound on show.

[vacdac]

Ta Serge,

Thats really helpful, I do seem to remember (dimly..lol )seeing this somewhere...it may actually have been you even in a previous post. But this "Kinda" chimes with my previous experience of bridging with my Audiolab 8000x7 , initially at lower volumes it had a little more authority, but at higher volumes it would cut out (thermal protection) at high but none stupid volumes. Interestingly it sounded best biamped but I think that this may perhaps have been down to some poor design decisions/choices by Audiolab (please be gentle with me here tho' as I do really agree that biamping is pointless in the face of simply getting a decent amp in the first place)

In a Hi-Fi choice group test a few years ago, the reviewer mentioned that the "output devices"(2 per channel) were mounted directly onto the heatsinks with very tight spacing between each individual channel. I always wondered whether if better spacing/larger heatsinks/deeper casework had been specced, whether the issues I found may have been ameleorated? I'm a little out of my knowledge zone here though, & would be grateful to tap into your knowledge, if I may? (for two channel use when biamped I had the inputs set up so that there was spacing between each driven channel on the heatsinks....done through the little known "scientifical" meffod of feeling wiv my hand ) also it ran cooler at sensible levels than when running just two channels.

The explanations I've come up with are completely speculative really, I don't mind being corrected & always willing to learn. .

Cheers Serge.

Regards Chris.

If you don't need the extra power then there's no benefit to bridging. When bridged, each half-amplifier sees half the load impedance, so it gives the amplifier a slightly harder time. I know you said that your 'speakers are an easy 8 ohm load, and the A500 is happy driving down to 2 ohms but nevertheless, if there's no need for the extra power, then I wouldn't bridge.

S.

[SergeAuckland]

Ta Serge,

Thats really helpful, I do seem to remember (dimly..lol )seeing this somewhere...it may actually have been you even in a previous post. But this "Kinda" chimes with my previous experience of bridging with my Audiolab 8000x7 , initially at lower volumes it had a little more authority, but at higher volumes it would cut out (thermal protection) at high but none stupid volumes. Interestingly it sounded best biamped but I think that this may perhaps have been down to some poor design decisions/choices by Audiolab (please be gentle with me here tho' as I do really agree that biamping is pointless in the face of simply getting a decent amp in the first place)

In a Hi-Fi choice group test a few years ago, the reviewer mentioned that the "output devices"(2 per channel) were mounted directly onto the heatsinks with very tight spacing between each individual channel. I always wondered whether if better spacing/larger heatsinks/deeper casework had been specced, whether the issues I found may have been ameleorated? I'm a little out of my knowledge zone here though, & would be grateful to tap into your knowledge, if I may? (for two channel use when biamped I had the inputs set up so that there was spacing between each driven channel on the heatsinks....done through the little known "scientifical" meffod of feeling wiv my hand ) also it ran cooler at sensible levels than when running just two channels.

The explanations I've come up with are completely speculative really, I don't mind being corrected & always willing to learn. .

Cheers Serge.

Regards Chris.

Heatsinking is really a long-term issue, in that the purpose is to remove heat from the output transistors as it builds up due to operating.The amount of heat depends on the standing current, which is why Class A amps run so hot, and the average, long(ish) term audio power. By longish, I mean over a minute or few minutes. Instantaneous high currents heat up the transistor junction, which can destroy the transistor if excessive, and as all heatsinks have thermal inertia, no amount of heatsinking will avoid short-term overheating. That's why amplifiers have overcurrent protection which shuts down the output transistors if they are being asked to give out excessive current which would result in local overheating and possible destruction of the outputs.

Sonically, heatsinking has no real effect, so putting bigger heatsinks on an amplifier won't affect how it sounds, just how reliable it will be in the long term, and how much sine-wave testing can be done before overheating, or how much power into severe loads the amp can take, again with reliability in mind.

If an amplifier is seriously under cooled, then yes, this can affect how it sounds because the outputs can overheat even with modest loads and powers, but no HiFi amplifier I know of ever got that bad. If you have a look at the tiny heatsinks that were used on the A&R Cambridge A60, or Sondex or QED amplifiers, these were perfectly OK for speech and music use, but were a bit small for long-term sine-wave testing. Of course that's not how these things are generally used so from the manufacturer's point of view, the heatsinking was adequate.

S.

[vacdac]

Ta Serge,

Appreciate that.

B.R.

Chris .

p.s it's a seven channel power amp I'm referring too. may that have had any bearing re it being undercooled?

[SergeAuckland]

Ta Serge,

Appreciate that.

B.R.

Chris .

p.s it's a seven channel power amp I'm referring too. may that have had any bearing re it being undercooled?

If the amplifier is undercooled, and a 7-channel Class AB amplifier is a lot more of a challenge than a standard stereo amp, then it could (should!) protect itself if being use hard. However, I don't see that sound quality would be affected to any extent, just reliability if the amplifier is being regularly stressed to the point that the protection acts. What might be worth doing is to locate a small quiet fan somewhere where the hot air from the amplifier's heatsinks is located to disperse the hot air. That should help a lot with cooling. It doesn't need much of a fan, just something that can stir the air and stop it gathering round the heatsinks. Heatsink efficiency depends on the temperature difference between the heatsink and the surroundings, so if the heatsink is surrounded by hot air, it won't do much cooling.

S.

[vacdac]

Thanks again Serge,
I no longer use it for stereo, as I got a new power amp last year, I do still use it for a.v. but only three channels as my pre has H.T. bypass. In its current usage I don't seem to have any issues re. heat anymore.

The only time I ever heard any hardening in the sound was when I ran it bridged, not when I biamped the main speakers, & given your previous statement that "bridging can give an amplifier a slightly harder time" due to seeing a lower load impedence ie halving, then this may make sense if I am following you right

What I did find though was that running the main speakers bi-amped rather than just driven by two channels was that the amp would run cooler at the same given volume. I did level match as I do have a decent s.p.l meter . As you say, implementing a 7 channel amp is more of a challenge! It's just that thinking about it has, had me puzzled from time to time It's always had good airflow all around too, front, top & sides..

Thanks for taking the trouble to answer my questions Serge I appreciate it.

Cheers.

Chris

[Hobbit13]

Clearly, it won't heat up the bulk of the magnet to anywere near the Curie point, but could it heat up a small section? Also, is the loss of strength progressive or an all-or-nothing? Is it unaffected under the Curie point, then drops off totally over it, or is there some degree of increasing loss?

I've read that the old fabric tweeters suffered from reduced output sooner when the "hoods" were not in place. The original poster pinned this down to the tweeter being very sensitive to dust. That doesn't seem very likely to me, I'd rather say they are sensitive to UV-radiation which dries out the fabric. (makes it brittle) Anyway the reason that the output of the tweeters lowers is more likely to be related to the fabric dome, because the tweeters failed often after only 5 to 10 years, which is much too soon for any magnetic reduction effect. (assuming people didn't drop them often :-) )

In general the metal dome types are more reliable, but I like the sound of the old frabric ones better.

By the way, I did some calculations on the original filter of the B&W 802s80 (which is very close to the 801), which might be of interest: http://audiokarma.org/forums/showthr...=403443&page=4

[Dr.Jones]

Just read through the whole thread, very interesting - some bits clearly over my head but well explained.
Gives me even more respect for people designing/testing and making loudspeakers which sound good!!

Out of interest, did you have to do much change to the eq for the Scalford show to compensate for the room change etc?

[SergeAuckland]

Just read through the whole thread, very interesting - some bits clearly over my head but well explained.
Gives me even more respect for people designing/testing and making loudspeakers which sound good!!

Out of interest, did you have to do much change to the eq for the Scalford show to compensate for the room change etc?

Minimal. At home at tried an 80Hz null of about 6dB as I have a peak in the bass at home. I didn't need that at Scalford as the ceilings were that much higher and the 'speakers were much further from the walls. Subsequently, I moved the 'speakers at home further away from the walls, and don't need the 80Hz null any more. Everything else stayed the same.


S.

[User211]

Serge, with your experience with the A500, would you think there is any benefit on bridging them? If you did not need the extra power, and the speaker was a relatively easy 8Ohm load would there be any gains?
I am thinking on re-visiting the A500, to use with the AE2's, and having them with a pre this time, as before, I thought it sounded awful when using the volume controls.

Gulp shock horror

That was my A500... TBH I am aware of the supposed problem with the pots, but I didn't notice any real problems with them. I had some fun with the A500 by leaving the pots fully open and using a cheap chinese valve headphone amp as a pre - until it broke. Typical super-cheap non-quality - but it actually worked much better than you might expect.

One more thing - a Parasound A21 does NOT sound like an A500. It is technically more competent - it measures better. I think Serge asserts there should be no audible difference as the A500 is good enough to be transparent. I think there is a substantial sonic difference between the two amps - although the A21 does deliver a lot more power.

[BoringSounds]

please don't talk less , i for one listen to what you say

I 2nd that, great read and insight. Makes me want to try the same thing in the future...

[SergeAuckland]

Further to all the above, I've had the 801Fs professionally reveneered as they were looking somewhat tired and even a bit scruffy, as those who saw them at Scalford might remember.

This is what they look like now:



They were done in Zebrano, by a local furniture restorer. Definitely keepers!

S.

[SergeAuckland]

I also have fitted new tweeters, and realigned the crossovers. These new tweeters were for the 801M S3, and are a few dB less sensitive than the older ones. The frequency response is still within a 2dB window, but they needed rather less EQ to get them there, and are a much better match to each other. Great that B&W still support 30 year old products!

As a further aside, I've remeasured my A500 amps, and now that I've got over the silly problem with my generator having 0.05% distortion, it's now 0.012%, much better! Using the repaired generator, the amp now measures 0.02% distortion, now much closer to the 0.01% spec. That was measured at 120 watts output into 8 ohms at 1kHz.

S.

[wee tam]

looking very nice serge

[BobC]

Just caught up on this, very nice indeed.

[Howlindawg]

The new veneer finish looks excellent Serge, great project!