STK4048XI with 2SA1943 2SC2500 pair

[penjoseph]

The potential of STK404x series are under looked due to their age factor.

I tried STK4048XI (fully complimentary NPN - PNP driver stage) coupled with extra four power transistors - 2x 2SA1943 & 2x 2SC2500 - in the output.

With this topology (Sziklai pair) to boost chipamp power one can extract good amount of power. My objective was to drive a subwoofer. Tested working right from +12 0 -12 to +60 0 -60.

Compared to TAS5630 Class D mono module I have the bass is more refined. The TAS is more boomy. In no way one can compare the efficiency because Class AB requires much larger heatsink.

Any opinions ?

STK4048XI datasheet : http://www.pablin.com.ar/electron/circuito/audio/amp205/stk4048XI.pdf

 

[penjoseph]

This is how I wired it up with an existing STK404x series common PCB. The power supply has 20,000uF per rail.

 

[penjoseph]

STK4048XI measured distortion is 0.008% between 20Hz to 20Khz at 150w 8ohm power.

Attaching a picture of the internal circuit. Driver stage marked in the box. One of the few STKs which has full complimentary driver stage. Though the internal driver stage is Darlington pair with the additional output transistors it works as Sziklai pair too.

 

[johnego]

Any opinions ?

What I don't like about the higher-end of STK chips is the input stage. I guess I would prefer to replace the input stage than to replace the output stage. I'm curious to know how good is actually the output transistors of the STK chips.


It was mentioned about good complimentary between NPN and PNP, which is very possible for devices in a chip and done in factory. I was very curious about this.


Even with regular STK chips with no pinout at the VAS, we can still modify the circuit as this is not really a chip but a 'hybrid'. But after I opened the case (the black plastic was removed), I couldn't find a good way to stick the module onto a heatsink.

 

[SVI2004A]

What I don't like about the higher-end of STK chips is the input stage. I guess I would prefer to replace the input stage than to replace the output stage. I'm curious to know how good is actually the output transistors of the STK chips.


It was mentioned about good complimentary between NPN and PNP, which is very possible for devices in a chip and done in factory. I was very curious about this.


Even with regular STK chips with no pinout at the VAS, we can still modify the circuit as this is not really a chip but a 'hybrid'. But after I opened the case (the black plastic was removed), I couldn't find a good way to stick the module onto a heatsink.

You’d like the Darlington Power Pack series and STK20** and 21** series
These are the output stages where you make the voltage amp stages

 

[johnego]

You’d like the Darlington Power Pack series and STK20** and 21** series
These are the output stages where you make the voltage amp stages

I have something similar, STK for driver stage and STK for output stage. How do the output STK compare with say the best Sanken mt-200?

 

[SVI2004A]

The big SanKens can’t be matched for current and power.. the way they’ll make sound largely depends on the circuitry driving them

But in the 60W-100W range some of those STKs are on par with sound quality with anything else out there, again front ending is the biggest variable

Technics SVI series is less obvious but top notch in sound...

 

[johnego]

The big SanKens can’t be matched for current and power.. the way they’ll make sound largely depends on the circuitry driving them

But in the 60W-100W range some of those STKs are on par with sound quality with anything else out there, again front ending is the biggest variable

Thanks, then I will try to use my output stage STK in my current amp projects.


AFAIR, the PDF for those STKs do not specify the fT, hfe, Cob, linearity or may be Ic. So I'm not sure how they perform in comparison with discrete transistors. But the idea that the chip has better matched NPN and PNP is interesting enough.

Technics SVI series is less obvious but top notch in sound...

Actually I owned the SVI2004. It's the 'BC Module' one. The bass quality is so very different. I didn't like the sound though (too 'hard') but I thought that with proper implementation it could shine but my plan was to crack it open and took the output stage because I don't like the STK input topology (LTP with current mirror to push THD down).

 

[SVI2004A]

Most of the sound the 2004 makes is a lot to do with the Voltage amp stage.
Technics used AN7062 as their go to voltage amp.. but the C version (BC module) was a far cry from the A and B versions. (No Synchro Bias)

Same drive circuit could be used on STK 2028~2068

All STK4*** have thevoltage amp integral - which you do not like
The DPP ones give you the freedom to create voltage amp how you like (could use AN7062 or make own discrete circuit)... can have mosfet differential stage etc

 

[johnego]

but the C version (BC module) was a far cry from the A and B versions. (No Synchro Bias)

I have guessed so. Thanks.

 

[penjoseph]

The reason why I chose STK 'hybrid' case is I have found that generally chip-amps produces less hum on open input as compared to entry level (meaning poor) discrete amp designs - observed it during my experimentation with LM3886 .

Was thinking of pairing the STK output with Sanken 2SA1494 & 2SC3858 . But the issue is the PCB with extra wide transistors will turn out to be much bigger.

In the STK sample circuit pin 6 from the input stage is left unconnected. Does anybody know its purpose ? diyaudio member@SVI2004A ?

 

[SVI2004A]

If I was going to use external transistors I’d definitely go to 1494/3858 even if larger heatsink area needed...

On a whole heap of STK40** series pin 6 is not connected- thought it may be used either in part of a muting circuit or for internal factory testing.

 

[penjoseph]

Finally managed to create dual side PCB drawing with the help of a designer.

 

[penjoseph]

Got few double-sided PCBs fabricated for testing. The first set was smaller (right) and couldn't accomodate MT-200 size transistor. Hence I had to make longer PCBs for the second set (left).

 

[penjoseph]

My first objective was to make a good subwoofer amplifier. Hence I increased the capacitor values in input stage & VAS restricting bandwidth.

After testing I removed the 2x 33pf capacitors in the feedback path because when it was tested with 100pf the DC offset increased to 40v ! It is not present in STK datasheet.

Increased power transistor base resistance to 2.2ohm. DC offset is currently stable at 14mv.

Updated circuit:

 

[Kay Pirinha]

How did you manage to match the auxilliary output devices close enough so that you can omit the emitter balancing resistors?
Best regards!

 

[penjoseph]

The load balancing resistors (0.22 or 0.1 ohm) are shared at the collectors of the auxiliary power transistors & emitters of STK. I didn't closely match external transistors (only checked for polarity & resistance between B C E) but having 2x parallel & STK controlling circuit stability I believe its not an issue.

I had DC offset of 14mv with only the STK chip & even after connecting the transistors its the same at 14mv. Voltage rails are 52+ve -0- 52-ve.

There are 4 power transistors per rail (2 Auxillary & 2 in STK). In fact I was thinking of having individual load resistors (4 per rail) but sharing a common resistor between a PNP-NPN pair worked fine - hence 2 each per rail.

This circuit produces Zero audible hum even at open input - (with the white 2.54m XH wire connector disconnected). It is very easy to build this powerful circuit provided the STK chip is good.

 

[Kay Pirinha]

Ok, thanks, now I see. It's Sziklai's, with their virtual emitters at pins 13, 16, 17, and 18.
Best regards!

 

[aparatusonitus]

It is very easy to build this powerful circuit provided the STK chip is good.

My research on the STK4048XI authentication showed that all STK4048XI chips that have 4271 prints in the upper right corner are actually STK4048V. You can check this from the back of the STK module ... if the code starts with E4048 (and I'm 99,99999% it will, sorry) ... you have an STK4048V whose output is quasi-complementary rather than complementary as in the original STK4048XI.

 

[penjoseph]

Well I tested the STK before soldering to the board. Transistors pin 13,17 to 14 had reverse continuity compared from 16, 18 to 12 - tested continuity with a multimeter.

This makes it a complementary.

I had made further changes in the circuit keeping the offest low & for a Subwoofer this architecture works much better than the cheap Class-D amps out there. Works right from 12v-0-12v to 80v-0-80v DC (depending on desired output & power supply). For less heat dissipation vs. power 60v-0-60v DC would be ideal.