Mod Upgrade of BK ST-210 commercial amp

[Grishnak]

Hi folks, so I'm a noob and have spent a few hours perusing the forums and researching terminology to familiarize myself with DIY amps with an eye towards actually building one ...

I sold the commercial stuff from 1980-82. Mostly low end but also some fairly nice SAE, Carver, Phase Linear (ka-boom!) and some decent reel to reels, etc., so I have a rudimentary knowledge of some of the basics.

With that introduction, my question is ... I have a BK ST-210 which I would like to (for my first tech effort) crack open and explore the need for upgrading or modifying components. Can someone help direct me as to what I should look for with this unit? I understand it is supposed to be rated at 300 watts into 4 ohm load. My current speakers (purchased new in '82!) are the AR 9LS which vary down to 3.2 ohm. Do I look for cap leakage? Should I just plan on upgrading the power cord? What else should I look for and is there any part(s) that I can replace (even if not 'bad') which will improve the sound/performance?

Thanks! This is my new favorite place in the virtual world.

Dan

 

[orbanp]

Hi Dan,

Welcome to diyAudio!
Before you start modifying your amp, read up a bit on audio amplifier design, as well as on audio myths.
Here are some good sites to start with:
The Amplifier Institute.
DIY Audio Articles
The second site also has an article on myths: Myths in Audio

Peter

 

[Grishnak]

Peter, thanks for the links! The myths article is just awesome. What a relief I don't need to go out and replace all my cables! Although I may make some shorter speaker wires. In the 80's we just tinned zip cord so I suppose that will still suffice. One item that's not addressed in the article is whether DACs are effective. I've been running my iPhone directly into my preamp and streaming Pandora. Any thoughts on DACs?

Thanks again! I think I'll refrain from cracking the amp open for now ...

Dan

 

[orbanp]

Hi Dan,

Myself, I am totally ignorant of DACs (from a user point of view). However, this is a good site to read about the various issues relating to DACs in audio systems:
The Extremist DAC
The same site also has a very interesting read, and designs, regarding thermal distortions in solid state amplifiers. If you are into audio circuit engineering this is worth reading:
Memory Distortion Philosophies
And the home page: My little Audio Site

Regards, Peter

 

[Grishnak]

Thanks, Peter. The links are interesting. I must admit that his design explanations go right over my head. I am largely ignorant of the amp language. I continue to read and check wiki etc. for definitions. My basic understanding is that iPhones, iPods, CD players, computer sound cards, all emply DACs but they are usually crappy which leads to incorporating better, strand alone, DACs into the amplifier feed. Makes sense. I saw a retail/commercial DAC yesterday for $20k. Seriously? I just want to insert a decent DAC between my iPhone streaming pandora, etc, and my amp. Lol

 

[sgrossklass]

These iDevices (first iPhone and first iPad possibly excepted) tend to have pretty decent DACs built-in. Usually low-power 24-bit affairs with 100+ dB SNRs. Nothing that would beg for immediate replacement for sure. They're just a little quieter than typical CD player level, about 1.2 Vrms or so.

Possibilities for amp upgrades are best discussed with a schematic of the unit at hand. (300W/4R suggests something reasonably complex. Unfortunately this amp seems to be quite exotic. Do try to obtain service docs if at all possible though.) Otherwise serious experience will be needed. It would also be useful to know a bit about the unit's history, particularly how it was treated. From what you write it probably saw ordinary domestic use only?

Honestly, cheap and effective tweaks tend to be rather less common. Of course you may see the odd intermittent switch or relay in need of cleaning, or scratchy pots, or the odd small dried-out electrolytic or cracked solder joint, or idle current being off, so restoring a unit to a state as good as new is quite doable. But better than new, that tends to get tricky unless someone really goofed at the time. In some cases you'll find an opamp that can possibly be replaced by a better one, but power amps tend to be all discrete affairs.

Preamps usually offer more tweaking options.

Incidentally, you could try running your iPhone's headphone out directly into the power amp. It's definitely worth a shot, they tend to be quite low in noise and all. I'd assume a fat pro power amp would have a gain of 30-33 dB, so you should be able to get close to max power if needed.

BTW, if the power amp has an input level control, this is usually best set all the way up or quite far down. Around the -6 dB point, distortion (along with noise) may rise considerably. Some Behringer amp (that Alesis "clone") showed like 0.3% then... ouch.

 

[Grishnak]

Thanks, sgrossklass! The BK amp is something I just bought. It's actually arriving today. According to the seller it was a home use amp in good condition. It was cheap enough that I haven't actually auditioned it yet lol. I know, crazy, but I read great reviews and it was a good price point.

The only problem with running the iphone directly into the amp is that it has only the two inputs which would then require changing out the interconnects every time I switch source, but it's worth a try to see if it improves significantly. Thanks again!

Dan