krell ksa5 klone

[paroxod4]

Please give a link to a good seller for 2n5564-5566 on ebay. Input assemblies affect the constant voltage at the output, the less it is without an integrator, the more likely it is that transistors are not fake. I have a lot of original BF256C, so I asked about the possibility of using them.

[Mullet]

Do you mean that the voltage across the test points will constantly change? In my case I set the voltage to 125mV via pot after the amp has been warmed up. It drifts a little but doesn't stay exactly at 125mV. Or do you mean the dc offset with OPA445 removed?

This is who I bought 2n5564 from...

https://www.ebay.com/itm/1PCS-New-2N5564-CAN-6-Matched-N-Channel-JFET-Pairs/392370230460?hash=item5b5b1694bc:g:wDEAAOSwvFNdSPV7

[Pars]

Not sure, they might work.

I took a look on diyaudio. Not much discussion or mention of the BF256C. I did find this:

alternative transistors, not as costly as the Toshibas SK170 and much easier to source could be the BF256C and the BF246A/247A. The former is a bit higher in noise (still sufficient for highlevel circuits), the latter features much higher Idss which allows for larger source resistor values and for higher bias currents, which could be advantageous in output buffers and buffers that need to drive capacitive loads -which is a very common case with filters.

https://www.diyaudio.com/forums/analog-line-level/169005-jfet-active-crossover-3.html#post2227453

EDIT: and from earlier in that thread from the same poster:

well regarding soundjudgements aren´t easy, because everybody has a different taste. To me all OP-Amp filters I built sounded artficial to a certain degree. I never doubted to listen to a reproduction of music. The first discrete JFET-filter changed that. Now there was music a lifelike quality which sounded authentic. Music instead of HiFi. I know that distortion-wise the simple Buffer is very good, but dosen´t reach a OP´s low values. But the...who cares about distortion figures, which -as we know for 30 years now- don´t correlate at all with the sonic impression? Noisewise even the BF256C settles around -100dB, the BF246A, 2SK373, 2SK170 beeing even better. So noise should be no big issue. Hum can be an issue in every circuit and typically it depends alot more on the circuits surroundings than on the circuit itself.

A different thread, but doesn't sound promising:

https://www.diyaudio.com/forums/pass-labs/145201-building-symmetrical-psu-b1-buffer-230.html#post2794067

I'd probably just go ahead and try them, observing pinout, but without the correct part or know equivalent, it might be hard to tell.

 

Edited February 14, 2020 by Pars

[paroxod4]

 

 

[paroxod4]

Mullet Input field effect transistors 2N5564 affect the constant component of the output of the amplifier, and not the quiescent current of the output stage.

[paroxod4]

Pars  Thank you so much for such a detailed answer.  Mullet  Thank you for the link. 

[Mullet]

I recently finished my Krell KSA-5 Klone. I'm having something weird that I can't quite wrap my head around. When listening to music there is a lack of bass. It just seems to be sucked out. I swap the same HD800S headphones into another amp and voila bass is there on any given track. I don't hear any distortion or a buzz or anything that would be akin to a ground loop. Anyone ever heard of this kind of phenomenon with this amp?

[jose]

Have you checked the ground?

[thuytn]

Sounds like out of phase problem. Check wiring.

[MLA]

Definitely not the case with the one I built, so something is off. Could be many things though  

Aside from out of phase driven cancellation, I wonder if this is unique to your HD800 or does the same thing happen with lower ohm headphones?  

[kevin gilmore]

one other person over there had the same issue, turned out to be a miswired 4 pin jack

[Mullet]

I initially noticed issue when both channels were out of balance. I first looked at the hp jack because I noticed it wasn't isolated from chassis ground. I've since fixed that issue. Then the sound got better. Both channels balanced with same apparent level, but with the lack of bass and what seems like a really wide stereo image. Wider than expected. It could be a phase issue. Would a grounding issue show up as phase problems? What's interesting is that it's only in the bass region.

Connectivity looks correct. I've toned everything out with a DMM. So I don't think I have anything backwards. IN1 is my left channel and IN2 is my right channel. Same goes for outputs.

Other things to note: I did have signal ground connected to the chassis then through a ground loop breaker. I haven't been getting hum so I've removed the GLB. Same issue. Using the GLB, It made the chassis share the same ground as signal ground. Not sure I'd want this... maybe I do?

I also originally only had OUT1, OUT2, and one of the GND paths going to my HP jack. I've since added the the 2nd GND path. No dice. Sounds the same.

I did buy 2N5564 on eBay. I know I prolly shouldn't have. Could poorly spec'd or fake 2N5564 create a situation where the bass is so rolled off?

FInally, trying different headphones didn't make much of a difference. I mean they sound different, but don't have bass.

I'm going to try a full rewire on the outputs to see if that makes a difference, but may look at replacing the 2N5564 next.

[Mullet]

Ok... I think I figured it out. I used Belden cable for my hookup wiring and it looks like ground and left channel were touching because a dodgy stripping job. So chalk it up to faulty cabling. Phew!

Edited April 1, 2020 by Mullet

[paroxod4]

Привет всем, пожалуйста, скажите мне, какие пары выходного каскада лучше выбрать, если это возможно по схеме.

Hello everyone, please tell me which pairs of the output stage are better to choose, if possible according to the scheme.

[paroxod4]

According to the scheme: Q23-Q24, Q25-Q26? Q23-Q25, Q24-Q26? Or pick up the whole four?

[alexcp]

I matched the quad of Q23-Q26 by hFE, and separately matched the octet Q15-Q22. However, my main reason for matching was because I could. You cannot get perfect symmetry between NPN and PNP transistors no matter what. In my case, I could not match MJE15030 with MJE15031 from the batch I had, so I replaced them with D44H and D45H, respectively - these matched much better.

[paroxod4]

The maximum that I could for the Kelvin board, 30/31 beta -122/136, respectively. There are 32/33 even worse, 113/179, but they are on a black expensive Chinese board, I bought it in the case, the sound is good.

[paroxod4]

 

[paroxod4]

 

[jp11801]

.

Edited June 21, 2020 by jp11801
Sobered up

[alexcp]

I built my KSA-5 clone back in 2013 (see my post dated August 9, 2013 in this thread on page 8) but was never quite satisfied with it. I made every effort to make it look good and perform well, and it was well within the original KSA-5's specs. Yet, although it worked well with my 32ohm Grado headphones, it could not compete, in my subjective opinion, with Musical Fidelity's X-CANv8. Connected to a pair of 8ohm speakers, the clone would become rather confused with anything but simplest music. Because of this, the amplifier fell into disuse and was gathering dust on my rack.

Until this weekend.

This weekend, I finally got around to make KSA-5 work for me. I kept the overall topology and the PCB, only changing some passive parts. The results are quite remarkable:

• The original KSA-5 was rated for 5W into 8ohm with THD<0.5%. Before modification, my clone gave 5W into 8ohm with THD @1kHz of 0.18%, well within the specs. The revised clone delivers 5W into 8ohm with 0.0015% THD, an improvement of more than two orders of magnitude.
• The original KSA-5 was advertised to deliver THD<0.03% into 100ohm load, although the brochure did not give the signal level for this performance. Assuming the same output voltage as for 5W into 8ohm, about 6.3Vrms @ 1kHz, my unmodified clone demonstrated THD of 0.02%, again within the specs. The revised clone drives a 100ohm load to the same level with 0.0017% THD, an improvement of more than an order of magnitude.
• The modified channel sounds very well, clear and transparent. I still need to complete the second channel and do some real listening.

I will post the revised schematic with some explanations, as well as additional measurements, in this thread.

[alexcp]

Let me start with a few measurements of the unmodified KSA-5. They seem to confirm the listening experience described above.

With a 100 ohm load, KSA-5 never leaves Class A, and the performance is decent, with 0.02% THD and dominant 2nd and 3rd harmonics:

[alexcp]

With a 32 ohm load, Class A extends to 6.4 volts peak, so at a lower level of 4Vpeak the performance is quite similar:

[alexcp]

With an 8 ohm load, Class A ends at 1.6V peak, so 1W performance is not so good anymore. Curiously, the Krell brochure mentions that the amplifier is good for 5W into 8ohm, but the owner's manual warns agains connecting it to any loudspeakers. I understand the 8ohm was meant for driving STAX via a transformer.

As usual, at a higher output level the distortion percentage improves somewhat:

[alexcp]

To see what and how to improve, let's have a look at the original schematic.

KSA-5 is designed along the lines of "moderate feedback", that is, it uses very little to no global feedback but lots of local feedback a.k.a degeneration.

The pair of input JFET buffers (Q1, red box on the schematic above) run independently of each other and outside of the global feedback loop. With low loop gain, they see very different signal levels, so the differential stage downstream doesn't cancel their distortion. (BTW, because of this JFETs need not be matched. Also, the expensive and hard-to-find JFETs can be easily replaced here with BJTs.) Still, a JFET follower loaded by a current source has 100% degeneration and relatively low distortion, at least at low signal levels, so the buffers are not the biggest problem.

The pair of differential stages (Q2+Q3, Q7+Q8, orange box) is heavily degenerated by 680ohm emitter resistors and produce R10/(R1+R2) = 2 = 6dB of gain.

The pair of common emitter stages (Q12, Q13, purple box) is also heavily generated by 402ohm emitter resistors and, with the low load of R23 and R24, provides R23/R16 = 9 = 19dB of gain.

Since the output stage (blue box) is a double emitter follower with approximately unity gain, the total open loop gain of KSA-5 is 2x9 = 18 = 25dB. The feedback divider (R45-R47) attenuates the output signal by a factor of 9 (19dB), which leaves 18/9 =2 (6dB) of global feedback. That is, the global feedback loop attenuates the distortion of the output stage by a small factor of 1+2 = 3.

The output stage, meanwhile, is a large source of distortion. Although Krell claimed that KSA-5 runs in "pure Class A", in reality it can easily slide into Class AB. The output pairs run at only 50mA of quiescent current each and leave Class A (that is, one half of the output stage stops conducting current) when the output current reaches 200mA. The driver quads (Q15-Q22) also run in Class AB (R37 and R38 are connected to the output), which means they stop conducting at that point, too. With a 100ohm load, it would happen at 20V peak output voltage, so the amp never leaves Class A with such a load. However, with 32ohm, KSA-5 leaves Class A at 6.4V peak; with 8ohm, at 1.6V.

Even within Class A region, the output stage is not very linear, especially with low impedance loads. It uses paralleled transistors with relatively large emitter resistors to ensure current sharing. The dark side of large emitter resistors is that they make the output impedance of the emitter follower large and nonlinear in the crossover region (see e.g. Douglas Self and his "wingspread" diagrams). Since the output impedance forms a voltage divider with the load, its nonlinearity makes the gain of the emitter follower nonlinear, adding crossover distortion and negating the benefit of the large bias current.

Overall, KSA-5 has a nice and linear front end followed by a not-so-linear output stage, with little feedback to let the former help the latter stay linear.

The game plan, then, is to improve the output stage and add more feedback.