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.

Velleman K4040 Photo Gallery

 This is one gorgeous looking amplifier!

Velleman K4040, take three

This is the last of three posts on Velleman K4040. Here are the links to the first and second posts.

Over 10 years ago, I built a Velleman K4040 power amplifier from an (expensive) kit. The amplifier still looks quite impressive:

Not satisfied with the out-of-the-box performance, I modified the amplifier (see my previous post for details). The result was a dramatic improvement in sound. However, without its global feedback, the amplifier had higher measured distortion, higher input sensitivity and more hum.

Ten years later, I (slightly) revised the amplifier. I replaced the resistors in the signal path with mil-spec metal film from Vishay, tidied up the wiring of the phase splitter and added a global feedback loop that encloses the input stage. With it, the amplifier has 0.015% distortion at 1W (that is, one-sixth of its original specification), the hum is much reduced, and the input sensitivity is in line with the output voltage of today's signal sources.

The final schematic:

Measurement results compare well to those of tube power amplifiers made by major brands and priced at up to $10,000, as measured by Stereophile. The modified Vellemn K4040 offers respectable measured performance typical of a classic tube design.

More photos (click for higher resolution):

DCPP aka Engineer's Amplifier

In early 2011, I built a Distortion Cancelling Push Pull (DCPP) amplifier designed by Peter Millett. It sounds very nice, was fairly easy to build, and is rather inexpensive. Thank you Peter for the great design!

Velleman K4040, take two

This is the second of three posts on Velleman K4040. Here are the links to the first and third posts.

The sound of Velleman K4040 as built from the kit (see my previous post) was not in line with the price of the kit, so I decided to modify it.

 

I found two posts on diyAudio.com by ecdesigns describing what he has done to K4040. (In case the first link doesn't take to the right place, look for posts #1029 and #1037 on the thread named "Building the ultimate NOS DAC using TDA1541A").

 

For your convenience, here are the list of the mods:

1. 100nF/63V capacitor across D23, this will suppress hum on the bias readout.
2. R5 and R10 lowered to 10K, C29,31 increased to 100uF/100V, 47V zener diode across C29 and C31 (cathode to plus), this will stabilize the bias voltage, regardless of transformer load.
3. High-quality potentiometers for bias adjustment (Bourns or Spectrol)
4. V11 changed to ECC83, improved sound quality (ECC83 from phase splitter can be used)
5. R61=100k, R62=100K, R59=27k, R60=27K, R11=6K8, R16=6K8, this will set the correct gain for V11.
6. C5=100nF/1KV, C9=100nF/1KV polypropylene (Farnell P/N 106367)
7. Cathodyne phase splitter replaced by modified Schmidt phasesplitter (see diagram). J508 and J509 are 2mA constant current sources (Farnell P/N 9549951). V9 changed to ECC81 (RS P/N 5011342).
8. C11...14, C19...22 changed to 100nF/1KV polypropylene (Farnell P/N 106367)
9. RV1...4, RV5...8 changed to Bourns or Spectrol.
10. R89...96 changed to 1K Ohm 1W.
11. R32...34, R51, R29...31 and R38 changed to 1.5 K Ohm.
12. Overall feedback removed, only using local feedback.
13. Svetlana EL34's replaced by JJ KT77, clearer sound, better bass and trebles.
14. 3.3nF/5KV capacitors were placed in parallell with D11...D14 to suppress switch noise.
15. ZD1=3.9V zener instead of 7V5 (solves mains brownout problem, this is already modified in new kits)

The new schematic is quite different from the original:

 

 

In fact, only the output stage with the quad of EL34 is left from the original design.

I implemented the proposed changes rather directly. The only difference is that I used IXCP10M45S current source instead of a pair of J508 current regulator diodes in each channel and replaced the 1N5408 rectifiers with UF5408 instead of adding noise suppressing capacitors in parallel to them. Here are some pictures:

 

The result was a remarkable improvement in sound!

 

In 2015, Mark Snape (a.k.a. SNAPEFU @diyAudio.com) made a PCB layout for this mod, although I have never seen it myself.

However, I still had two problems to resolve: (1) I got hum that wasn't present before the mods; and (2) a two-year old running around doesn't get along with eight hot, attractively glowing tubes. The project was shelved until a better time, which happened to came only in December 2020.

Velleman K4040, take one

This is the first of three posts on Velleman K4040. Here are the links to the second and third posts.

My second amp was the big, hot and heavy K4040, built from a Velleman kit. The kit is not available anymore, but the amplifier still looks quite impressive:

 

Two quads of EL34 tubes, one per channel, in ultralinear connection provide 2x90W of power into either 8 or 4ohm load. The power and output transformers are large toroids:

The amplifier also has a nice bias adjustment system for the eight power tubes - behind the front panel, there is a line of LEDs showing the bias level of any tube, one at a time, and trimpots for adjusting.

Features:

• Pure valve sound with high quality EL34 valves. 􏰄 High quality black and chrome housing
• Chrome valve socket covers
• Easy bias adjustment with LED indication
• Removable bottom for easy access and service. 􏰄 High quality capacitors and components
• Gold plated input and speaker terminals
• Standby function
• 􏰄Soft start circuit for power transformer

Specifications:

• 􏰂2x90Wrms into 4 or 8ohm
• Up to 2 x 15Wrms full class A
• Bandwidth: 8Hz to 80KHz (-3dB/1W)
• Harmonic distortion: 0.1% @ 1W/1KHz
• Signal to noise ratio: >105dB (A weighted)
• Input sensitivity: 1Vrms
• 115VAC or 230VAC / 500VA max.

The schematic of the amplifier itself is quite conventional - two stages of triode amplification, followed by a cathodyne phase splitter, followed by a power stage with four power pentodes running in ultralinear mode with fixed bias:

The power supply includes many nice features, such as soft start, stand-by mode with input muting, and the above mentioned power tubes bias monitor with LED indication:

For reference, the toroidal output transformers of K4040 have approx. 1.8k Raa primary winding (15:1 turns ratio, anode-anode to 8ohm) with 33% screen (ultralinear) taps, and a 8ohm secondary with 4ohm tap (which is also used for feedback). The DC resistance of the primary is 41ohm anode to anode (20-21ohm B+ to each anode) and about 7ohm from B+ to each grid.

Contrary to some reviews, I found this amp sounding rather nice, particularly after I replaced the EL34 tubes in the output stage with separately purchased matched quads.

However, the sound did not quite match the price, so I modified this amplifier, leaving little of the original design.

S5 Electronics "L" series tube amps

In 2009, I started with a Model 8LS stereo amp from S5 electronics.

Model 8LS uses a 5670 double triode as the voltage amplifier and phase splitter and two 6005 pentodes in the PP output stage. (S5 doesn't make the schematic public, so I won't publish it here. Should you want a look, search for it online.)

Initially I built it on a wooden plank as suggested by the manual but later moved it into a Hammond enclosure and replaced the transformers with Edcors (XPWR085 for power and two GXPP10-10K for outputs).

Separately, I built a Model 16LS, which is the same thing with a doubled output stage (four 6005) but the same output transformers as in the 8LS.

Both versions are very easy to build, nice with vocals, but not quite HiFi - see the FFT charts.

Matching (preferably curve matching) output tubes helps a lot with reducing distortion - helps way more than replacing the transformers or the electrolytic capacitors in the power supply! One can get these pentodes in matched pairs and quads from eBay; 6AQ5 or EL90 are other names for the 6005. Also, I am guessing that fine tuning the phase splitter and the screen grid voltage would improve the distortion a bit, although it requires at a minimum a sound card and an appropriate software such as the Arta.