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Old 9th May 2002, 12:51 PM   #1
Vivek is offline Vivek  India
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Default STK chips

Are the STK series power amplifier ICs any good at all, maybe for a subwoofer amplifier.
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Old 9th May 2002, 01:47 PM   #2
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Hello Vivek,

Modern STK amplifier modules are indeed quite good.
The specs are good but the modern high power models are not rugged - ie they are usually rated for 8 ohm load only, and if running full rated supply rails, a 4 ohm load will likely kill them.
(I get to repair plenty of such amps that have failed because youngsters have connected a extra pair of cabinets !.)

For economy, repeatability, servicability, availability and ease of implementation they are probably hard to beat.
Some have handy features such as muting and limiting also.
STK modules are pretty much standard equipment in a lot of mid-fi audio gear.

Typical applications use an under rated transformer to give high transient ability, and power rails that sag somewhat according to output, thus giving a level of protection to the module in domestic audio gear.
Adequate heatsinking is important, fan cooling even better, or place the heatsink in the air path of the tuning port tube.

I have repaired subs using stereo modules configured in bridge mono mode, and performance/price work quite well.

Although I have not tried the experiment ( I have 2 STK4048mk2 modules in stock to try out one day) I expect/suspect with good layout, good power supply and good input drive stage they ought to perform rather well.

Another approach is to use STK DRIVER modules driving seperate output transistors for higher power and/or lower impedence loads.
The specs for these modules is rather good also.

For a bi-amp/tri-amp loudspeaker, these modules would provide a quick, easy, economical and easily serviced soloution.
I believe some modern active cabinets are indeed configured this way.

If you build such an amplifier, please report back your experience.

Regards, Eric.

PS - such an amplifier maybe the basis of another 'Gain Card' ?.
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Old 9th May 2002, 05:03 PM   #3
Vivek is offline Vivek  India
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Hi Eric and thanks for the detailed reply.
I bought a STK 4171 Mark II amp kit. I bought it because I needed an amp to test a pair of bookshelf loudspeakers I built (my other audio system is too high powered for the new speakers and I am taking no chances). It is very cheap too. Of course, I do not think it is for serious hi-fi.
I am building the 60 W (Project 3A) amp by Rod Elliott.
The STK kit is almost done and I shall be more than glad to report the results to you.

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Old 9th May 2002, 06:21 PM   #4
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Vivek, Hiya again.

I think you will be surprised how good STK's can be.
They have the advantage of close thermal coupling, and small loop areas, and short conductor runs internally.
Pay close attention to grounding techniques, and psu decoupling and you will have a high power op-amp.

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Old 10th May 2002, 12:20 AM   #5
rljones is offline rljones  United States
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It is interesting that you should make this post, as I was about to start a thread on these hybrid op amps.

A few months ago, I made a pcb for the STK4048XI. [As an aside, this device has full complementary outputs; the 'V' versions, as opposed to the 'XI' versions, are quasi-complementary. A smaller module, the STK4044XI, has 4 output devices and 15 pins; the STK4048XI, 6 output devices and 18 pins. You can ignore the last 3 pins on the '48 if you put a '44 into the same circuit, so design for the 18 pin versions and you can go with either unit.]

Anyhow, I had so many problems that I became frustrated and put them away: horrible oscillation that I could not stop despite trying all sorts of things with the grounding, etc. However, after a friend of mine fried one of my amps, I decided to attempt making a couple of STK modules to put into the damaged unit. This weekend, I finally figured out the problems, one of which is hinted at on the application docs by Sanyo, and another which I accidentally discovered and yet seems very important.

The main secret in stopping oscillation is to keep the gain at 40dB. I don't like amps with this much gain as the input sensitivity is around 0.4 V for full power. This kept me from getting rid of the oscillation as I had initially changed the input resistors from 1K/50K to 1K/20K, and never looked back.

Once you get the gain up, to the suggested 40 dB range, almost all oscillation stops. The amp then looks very good on the scope, except for a little bit of something on the sine waves. (BTW, I used all of the parts suggested on the app docs, various 100pF caps, etc to avoid oscillation as Sanyo warns.) This where the application docs stop. What I found was that there was still oscillation, but at VERY low levels, like 1 mW output into 8 ohms. [And I'm now wondering if this low level oscillation is what has given these guys a back name, particularly with respect to longevity. Any amp that oscillates will overheat and eventually burn-out the output devices.]

At these low levels, the oscillation took off again. I tried all sorts of stuff to fix it: bypassing the feedback resistor with small caps like 10 to 200 pF, changing the input blocking caps, but nothing helped--until the other evening. I was looking at th scope ready to go to bed when I started holding a 200 pF cap across the feedback resistor when the oscillation completely disappeared. But I'd made a mistake, it turns out I'd actually put this capacitor from the amp output side of the feedback resistor to pin 3 going to ground.

In other words, by placing a 200 pF cap from the amp output to ground (but at low current level location; I didn't use the output terminals), the remaining oscillation stopped. The amp then looked absolutely perfect and all distortion dropped 10 dB, especially at low levels. The heatsinks were now cold when idling; before they were warm but this was due to overlooked oscillation.

This STK4048XI produces, with a +/-55 V supply that sags to 50V at full output, 140 W into 8 ohms and 220 W into 4 ohms. Freq response is -0.4 dB at 10 Hz to -2 dB at 100 kHz (limits of my equipment). Distortion (THD) is 0.8mW is 0.08% at 1kHz, 0.025% at 1W, 0.007% at 10W. At 20 kHz and 10W it is 0.035% and only 0.1% at 10W at 50 kHz.

As for sound, it initially sounds very good, not as open or airy as my Rowland amp, but very good. The input needs adjustment: my initially listening was done with an input transformer, which I think may be a mistake on this amp (I was still trying to cut that incredible gain...). Once I get it going, and properly set up, I'll compare it to a Tripath 104, a Rowland, and a homebrew Aleph.
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Old 10th May 2002, 03:13 AM   #6
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Thanks rljones,

for the words of your experience so far.
In my experience with domestic gear using these is that they are completely reliable, except for when loaded with too low impedence.
Typical amps strangle them with underrated power supplies - perhaps this is part or the trick to get them to run successfully.

And I'm interested to learn of your final results.

Regards, Eric.
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Old 10th May 2002, 03:18 AM   #7
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Default Additional Thoughts

Hello rljones again.
my initially listening was done with an input transformer,
Are you loading the secondary correctly to avoid peaking response ?.
Shunt load and build out resistors are required IMO.
I have had bad experience with a transformer input coupled amp until I did this.
Maybe this helps you, maybe not.

Regards, Eric.
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Old 10th May 2002, 04:04 AM   #8
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These amp modules work very good. I made once a 5 channel surround system with these. Oscillations can also start from bad grounding because of the high gain. Keep input ground, output ground and feedback ground in a star formation to avoid any loops or just keep your gain low.
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Old 10th May 2002, 04:59 AM   #9
mlloyd1 is offline mlloyd1  United States
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The recent STK modules do "function" well and are relatively easy to use, once you get the hang of them. However, I've never been a big fan of the resulting amps. I've suspected the lack of degeneration resistors in the diff stage is a big contributor to my displeasure with the sound.

By the way, here is a nice link to a bunch of data sheets for the STK modules as well as other discrete transistors from Sanyo.

I wanna' be like Harry when I grow up

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Old 10th May 2002, 08:16 AM   #10
Vivek is offline Vivek  India
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Does anybody have an idea about the distortion figures for an STK 4171 Mark II module? Some links would be useful.
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