Burning Amp BA-3b (Balanced) Updated Power Supply

In my previous post of the Burning Ampifier 3 Balanced, I mentioned that I replaced the CRC filters in the power supply with 100mF + 10mH + 100mF CLC filters per rail per channel. Here are some pictures of the new power supply:

From left to right, you can see two toroidal power transformers with a soft start mounted on top; four bridge rectifiers; first four capacitors, one per channel per rail, with their discharge resisotrs; four chokes; the other four capacitors; and the PCBs with the from ends, mounted on the rear wall of the chassis. On the far side, there are six pairs of power MOSFETs mounted on a heatsink. Here is a close-up shot showing how the capacitors and chokes are mounted:

The capacitors are Vishay/BCcomponents MAL2 101 16104, chokes are Hammond 159ZJ. The power transformers are from Antek.

Krell KSA-5 Clone - Revised Schematic and Build Guide

In the previous two posts on the topic, I discussed my approach to upgrading the KSA-5 and demonstrated the massive distortion improvement I was able to achieve. 

Without further ado, here is the revised schematic:

The list of parts required to modify two channels:

1. 8x 0.22 ohm 2W or 3W resistors
2. 4x 22 ohm resistors
3. 6x 47 ohm resistors
4. 8x 100 ohm resistors
5. 8x 274 ohm resistors
6. 4x 332 ohm resistors
7. 2x 562 ohm resistors (not needed if you can reuse R5/R8)
8. 2x 3.92k resistors
9. 2x 1 Megohm resistors
10. 2x 68pF 50V capacitors, NP0/C0G ceramic or mica
11. 4x 4.7nF film capacitors
12. Some 18 AWG single core insulated wire and 2x 10 ohm 2-3W resistors for the output RL network

The list of changes to the original schematic:

1. Replace R1, R2, R6, R7 with 100 ohm resistors
2. Replace R5 and R8 with 332 ohm resistors
3. Replace R9, R10, R11, R12 with 274 ohm resistors
4. Replace R16 and R17 with 22 ohm resistors
5. Replace R19 with a 562 ohm resistor (reuse one of R5/R8)
6. Replace R23 with a 1 Megohm resistor
7. Replace R24 with a 68pF 50V NP0/C0G ceramic capacitor
8. Replace R33, R34, R35 and R36 with 0.22 ohm 2W or 3W resistors
9. Replace R37 and R38 with one 47 ohm resistor connected between bases of Q23/Q24 and Q25/26. Make sure to connect the new resistor correctly - see the build guide below - or your output transistors are at risk.
10. Replace R47 with a 3.92k resistor
11. Remove C2 and C3
12. Add a 4.7nF film capacitor and a 47 ohm resistor, connected in series, between the collectors of Q2 and Q3. Add another 4.7nF film capacitor and a 47 ohm resistor, also connected in series, between the collectors of Q7 and Q8. Place the new parts on the underside of the board if you like.

Repeat for the other channel. That's it!

As resistors's names are not marked on the board, here is a photo showing what to replace with what. Note that the additional parts from step 12 above are not shown - they are under the PCB. Click the picture to open it in full resolution.

After assembly, take the usual precautions before powering the amplifier up, as if it were a newly assembled board. Turn the bias adjustment trimpots all the way counterclockwise, connect a current limited +/-21V power supply with the current limit set at 0.5A per rail.

With power on, check the output for a possible oscillation, then adjust the bias and re-check for oscillations. The bias level needs to be at about 20mA per transistor - with 0.22 ohm emitter resistors, it corresponds to 4mV between the test points, which should be easy to measure with a DVM. Higher bias levels are possible, but the distortion will be slightly higher. 

Let the amplifier warm up for 10-15 minutes and readjust the bias - it should go down as the output transistors warm up.

As with any feedback amplifier, capacitive loads may affect stability. In my testing, the amplifier remained stable with capacitive loads of up to 100nF. Consider adding the usual RL network between the output of each channel and the load to ensure stability. Make 20-30 turns of 18 AWG single core insulated wire on a 1/2 inch (12mm) former - a Sharpie will work - to make an air core inductor, then connect a 10ohm 2-3W resistor in parallel to it.

Krell KSA-5 Clone - A Massive Performance Upgrade

In my previous post on the topic, I reviewed the original schematic of Krell KSA-5 and concluded that is has a nicely linear front end that cannot help the output stage stay linear because of low feedback.

My plan was to improve the linearity of the output stage and wrap it in a feedback loop with as much loop gain as possible. An additional self imposed limitation was to keep the PCB and the general schematic intact as much as possible and use the parts that I had readily available, so as to complete the upgrade over one weekend.

I decreased the emitter resistors of the output stage by an order of magnitude, converted the output drivers to Class A, decreased the amount of local feedback (degeneration), dramatically increased global feedback loop gain - the revised KSA-5 has at least 50dB of loop gain over all audio range - and adjusted the compensation to keep the amplifier stable. 

It sounds immaculate and is a massive upgrade over the original version. I have heard it so far driving Sennheiser HD595 and Grado GS1000 headphones and B&W 602.5 floor standing speakers, with great results on a variety of music material. A brief head-to-head against Musical Fidelity X-CANv8 revealed that the amplifiers are somewhat different but without a clear preference one way or the other.

Below are some measurements. I believe that at this point, the distortion performance is largely limited by the original topology, such as the JFET buffers outside of the global feedback loop (note that First Watt B1, which is a JFET buffer, has similar distortion at similar input levels), and by the PCB layout.

1kHz THD and 19+20kHz 1:1 IMD @ 4Vpeak (1W) into 8 ohm:

9Vpeak (5W) into 8 ohm:

4Vpeak into 33 ohm:

9Vpeak into 100 ohm:

The amplifier clips nicely:

and delivers a good looking square wave:

The phase margin is more than adequate:

Krell KSA-5 Clone - Review of the Original Schematic

In the last post, I demonstrated the measured performance of my KSA-5 clone before the upgrade, which was not so great. Let's see why it distorts.

KSA-5 is designed along the lines of "moderate feedback", that is, it uses very little to no global feedback but lots of local 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.) Having said that, a JFET follower loaded by a current source has 100% degeneration and 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 have R10/(R1+R2) = 2 = 6dB of amplification.

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 amplification.

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 the biggest source of distortion. Although Krell claimed that KSA-5 runs in "pure Class A", in reality it is 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 - that's 320mW! No wonder the owner's manual warns not to connect this amplifier to any loudspeakers.

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 not-so-linear output stage and a nice and linear frontend that doesn't help the output stage to stay linear.

In the following posts, I will show how I improved the distortion performance of KSA-5.

Krell KSA-5 Clone - Measured Distortion Performance Before Upgrade

The measured distortion performance of the unmodified KSA-5 clone confirmed what was clear in listening sessions. It works ok with 100ohm and 33ohm loads, but is completely confused with 8ohm (click to open larger pictures):

Even with a 100ohm load, the distortion at 1kHz is 0.02%, or -74dB, equivalent to 12 bit resolution. 

In the next post, I will look at the reasons for this performance.

Krell KSA-5 Clone

When I saw, back in 2012, Kevin Gilmore publishing the blueprints for a clone of Krell KSA-5 headphone amplifier on head-case.org, I thought it may be a nice use for the bag of 1000uF capacitors I had at the time. (Yes, using up the capacitors was my main motivation for putting my Krell KSA-5 clone together.)

I made every effort to make it look and perform well. I carefully matched the transistors, used the best parts available, and even asked the people at Modushop to CNC a custom machined and anodized front panel.

Although good looking and well within specifications of the original KSA-5, the clone was rather disappointing. It was fine working with headphones, but it could not compete with my Musical Fidelity 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.

For the longest time, I wanted to make it work better but could not get to it until this weekend.

I decided to improve the performance of my KSA-5 clone without changing its topology, so as to keep the PCB, and to keep as many of the original parts as possible. Within a day, I was able to make quite some improvements:

• The original KSA-5 was rated for 5W into 8ohm with THD < 0.5%. My unmodified 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 would demonstrated THD of 0.02%, again well 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

I made some measurements under different setting, including with a 32 ohm load and with 19+20kHz two-tone test signal. I will post them and the changes I made in the schematic in subsequent posts. Stay tuned!

Burning Amp BA-3b (Balanced)

Big, hot, heavy, and definitely a keeper. The discussion of the amplifier is on diyAudio.com.

The build is in a 4U/400 case from modushop; each side has two 200mm heatsinks, each holding six MOSFETs (three complementary pairs) and a biasing circuit.

The construction is dual mono, with separate transformers for each channel. Power supplies occupy most of the chassis, while the actual electronics is mounted on the sides.

The power supply was initially CRC filtered, with four 22,000uF Mundorf MLytic® HC High Current Power Caps per channel (pictured).

After successfully fitting my ZenV4-J with CLC filters, I learned how much can be gained by improving power supplies in no- and low-feedback amplifiers, which  have little or no control over output errors and thus poor PSRR. On this premise, I replaced the CRC filters in the power supply of my BA3B with CLCs, so instead of 22mF + (2 x 0.22ohm) + 22mF I now have 100mF + 10mH + 100mF per rail per channel.

The power supply provides +/- 18V rails, with quiescent current set at 3 amps per channel.

Yet another Super Gain Clone

Unsatisfied with the sound of my gainclone amplifier (see an earlier post), I re-used the enclosure and the power supply for a gainclone along the lines suggested by Bob Cordell, whose implementation of an LM3886 based amp was praised by at least one member on the NJ audio society. Quote:"Bob showcased the Super GC at the NJ Audio Society and it sounded fantastic."

I skipped both the Klever Klipper and the toroidal air core output inductor, and kept only 10,000 uF per rail in the PSU. The schematic can be found in Chapter 27 of Cordell's Designing Audio Power Amplifiers. The PCB was designed to re-use the existing mounting holes of the ChipAmp's PCB.

The result? Better than with a plain vanilla chip amp, but IMHO still not good enough for music. Perhaps I should not have limited myself to re-use of the PSU et al. but should have taken all the details of my implementation seriously.

That was 2011. Looking back from 2019, I know I can do better! For more details, check out this discussion at diyAudio.com and my boards on sale.

 

Burning Amp 1

To get warm and comfortable on long winter evenings, in the end of 2010 I built myself a Burning Amplifier 1. With its 300W quiescent dissipation and ineffable ((c) Nelson Pass) sound, it is a nice winter time companion.

The enclosure, heatsinks and fans, power supply and speaker protection were salvaged from  my previous (not documented and hence not posted) attempt to build a Krell KSA-50 clone. The Krell clone worked but sounded strange, and I did not have the skills at the time to make it right.

I used four heatsinks, each holding two IRF250 MOSFETs in TO-3 packages and a PCB. Each pair of heatsinks is cooled by a quiet 140mm fan. Power supply uses a 400W toroidal transformer and 2x40,000 uF per channel; it is a dual mono configuration. The amplifier is housed in a 5U 400mm deep enclosure from modushop. Their "pierced" (perforated) base plate was very handy to keep all the parts together without sacrificing the looks. Total weight is about 50 pounds.

The knob on the front panel was designed as volume control, but it looked ugly, so I later remove it and replaced with a ON/OFF button.

LME49830 + 2SK1530 + 2SJ201

This was an implementation of National Semiconductor's reference design for LME49830 (now obsolete) with two matched pairs of Toshiba's 2SK1530+2SJ201 (also obsolete) per channel. As National Semiconductor in 2011 became part of Texas Instruments, this became a truly obsolete build.

The design is documented in two National Semiconductor's application notes, AN1849 for the power supply and AN1850 for the amplifier - the latter is unavailable from TI. The PCBs were made using National's gerbers for the amplifier and the power supply, thus National's logos.

The amp sounds surprisingly good, so good that I rebuilt is as a pair of monoblocks, each delivering healthy 200W into 8 ohms. It now looks much tidier; I will post the pictures of the monoblocks later.