AudioSource Amp ONE amplifier question.

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Hello, I could not find any thread on this topic. I recently bought this amp from my friend. I did some slight modding: change binding posts, put new output wiring( single wires from Kimber 4 speaker cable), new power plug. I drive it directly by Rotel RCD-955 AX CD Player through AudioQuest Ruby interconnects. This amp has passive input stage with its own volume pots. The amp drives PCB Alpha speakers. The problem is with lack of bass. From other people opinions ( other forums)it should sound better than my Onkyo integrated amp. Currently, there are good details in highs, midrange is not too bad but, with lack of bass sound is rather flat. Also, PSB speakers are easy to drive and they are capable of reproducing lows very nicely. I must admit, I am not bass hungry. I would like to have just a little bit more lows enough to fill the music. Is this problem related to lack of preamp? I do not know what to do. Currently, I do not have any pre in my system. I would have to build it. The amp by itself is not build bad. It has hefty toroidal transformer, Toshiba transistors........ I bought it for $60 .......with an idea of driving it directly by my player. I expected better quality of the sound...
I would appreciate any comments,
By coincidence, I just last week came into possession of two dead ones. I have not heard one play, but the build quality does not convince me that they are aimed at serious listeners.
The transformers are indeed somewhat heftier than I would expect, and there are a few film caps scattered throughout the circuit, but the heatsinks are absurdly small for an 80W/ch amp. This tells me that the bias current is next to nonexistent.
Lack of a preamp will not have any effect on bass, unless you intend to turn up the bass control, of course, but that's another question entirely. The only thing you might lack is gain, and that depends pretty much on the output level available from your CD player. Gain (again, assuming that you don't mess with tone controls) will only make it louder--at least up to the point where you run out of the 80W. As I noted, I have not actually heard one of these things so I can't comment directly on sound quality.
As for random peoples' comments on sound quality that you might find on the web...caveat emptor.
One thing that you might try if you're modifying the unit: Tag a little more capacitance into the power supply. If I recall correctly, there's only 10000uF for each rail and an open space just forward of the transformer. See if you can sneak a few caps into that space and connect them to the rails.
Given the tiny size of the heatsinks, I'd make sure that they're well ventillated. Don't stack them with other equipment if you intend to play them hard. Those heatsinks will need all the cool air they can get. The ones that were given to me died because they burned to a crisp. I haven't looked at them very hard, but one of the circuit boards is toasty brown all over. The other is cooked in the area near the heatsinks.

Hello Grey, thanks for the response. Yes, the same thing cought my attention after I opened the unit: two 10k uF caps & two
five-transistor banks attached to two small heatsinks. I bought this amp for two reasons: chassis and toroidal transformer. I do not think it makes any sense to spend more money/work on this project. Since I am not highly advanced in electronics, how to check parameters of the transformer? I wonder, would it be enough to power Mr. Pass BOSOZ preamp?
As long as you are going to spend some money, consider getting a RadioShack SPL meter and a test disc. Then with a little patience and graph paper tou can draw a frequency response curve. Do this for the old amp then for the new one. The purpose is to isolate the cause.

For instance an exaggerated (boosted) high freq range can sound like weak bass. It's very hard to tell the difference just by listening.

Another thing to watch is to be certain that when you swith amps, you don't move the speakers. Not even a fraction of an inch! Surprisingly small differences in position can alter freq responce substantially.

Ditto regarding the measurement position. Even in a listening test you have to watch this as sitting in a slightly different position can alter the appearent results. If you can't lock the SPL meter in to one position, an alternative and perhaps better way is to use several positions and average the result.

Since virtually all modern SS amplifiers sport flat resonse curves, these resons for poor response are usually (but not always) elsewhere. It's best to eliminate the more likely possabilities before spending your time, money and frustration on the less likely suspects.

If it turns out to be the amp, include in your list of suspects the input area. Decoupling caps and protection circuitry may (I emphasise, may) be attenuating the bass a little. A couple decades ago this was called an "automatic, built-in rumble filter"!! Befiore messing with it a schematic should be consulted.
Here's an 'off the wall' suggestion - a little effort/cost with a big result.

IF you have a low source impedance AND IF you have clear access to a conventional diff'l feedback front end, try this -

an active Q=2 filter built around the diff'l input as the active device. Set at the correct (for your speakers) Fo you can have better, more extended and cleaner bass, all for the cost of 4 film C's and 4 resistors.

Q=2? Yikes! Better make sure the woofers can take it first...
As for the transformer, it will make a dandy preamp power supply. I seem to recall that I measured it at something like 40-0-40VAC which is a pretty good voltage range for a preamp transformer. Assuming that my memory of 40V is anywhere near correct, the unregulated rail voltage will come in at about 55V or so. Now, the BOSOZ specifies a +-60 regulated rail, but it will work quite nicely on +-55V unregulated. Or, as an alternative, you could use a voltage doubler (see Nelson's A-75 project for an example) to increase the rail voltage. At that point it would be above the 80V unregulated voltage in the BOSOZ, but that can be easily handled.
Note that the heatsinks, although undersized for an amp, will be just marvellous for a preamp. The metalwork will allow use of the heatsinks after the circuitry has been removed, so that shouldn't be a problem.
Heat the soldering iron...there's work to be done.


P.S.: Work carefully removing the amp circuitry and set it aside intact. Make detailed notes on what hooked to what and how it all went together. (I've already removed the stuffings from the two I have, so I know this can be done fairly easily.) The build is modular enough that you can turn the thing back into an amp if you change your mind at some point in the future. These two are absolutely dead, so I know I won't be doing so with these, but you might want to keep that option open. All it will cost you is a couple of power supply caps to replace the ones you might have thought to salvage from the circuit, but will set you right on replacement caps for only a few dollars. Tell Steve that Grey sent you. After that, you're off and running.
I have SPL meter from RS. I bought it last year and had no chance to use it. And, this is another question: how to use it ? :angel:
Manual which came with it does not specify how to use it for testing. I did try once to browse the net for ideas but with no avail. I have also three Stereophile's test disk and one by Chesky.
Oh well, I am a ................................beginner in this great hobby :(
Hello Grey, this is a great news. I wanted to build this preamp since the circuit was published for the first time by Mr. Pass. When I first saw this amp, I thought about converting it into preamp if the sound would not be good.
Thanks for the link, I will use it:devilr:
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