HeadWize: DIY Workshop > β22: A discrete, cascoded, fully complementary, pure class A amplifier (part 3)

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DIY Workshop » β22: A discrete, cascoded, fully complementary, pure class A amplifier (part 3)
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set300b

HeadWizer

Joined: Aug. 1, 2001
Locale: San Francisco, CA, USA
Total Posts: 151

Message [#120] posted on: 10-31-2008 04:20 PM CST (US).

[Quoting mike b]
Very nice set300b.
Seems to me a tidy two box solution is The Standard for balanced B22's.
Where is that enclosure from? Is the top custom?

Thanks!
The enclosures are Context Engineering. Yes, custom tops.

housing

HeadWizer

Joined: Dec. 16, 2006
Locale: N/A
Total Posts: 163

Message [#121] posted on: 11-06-2008 11:08 AM CST (US).

[Quoting amb]
I posted this on the other forum and thought I'd also duplicate it here because it is of general interest for those who have or plan to build a 4-channel balanced β22 amp.
The standard 4-channel configuration has two 3-pin XLR balanced inputs, which requires the use of a source with balanced outputs. Indeed the balanced β22 maintains the balanced operation from the the input to the transducers only when used with a balanced source.
Sometimes you may want to connect a source that only has unbalanced outputs (e.g., a CD player, tape deck or whatever). This is where an extra set of unbalanced RCA input jacks and a unbalanced/balanced selector switch comes in handy. But since the balanced β22 will accept only balanced inputs, before you could switch anything, you need to convert the unbalanced source' signal to balanced. This is because the inverted "cold" signal for each channel is missing. An active unbalanced-to-balanced line driver circuit (based on something like the DRV134 chip), or a passive transformer made for this purpose (from Lundahl, Jensen, Cinemag and others) must be used in order to derive that signal. This will then allow the balanced headphones to remain "balanced" (i.e., hot and cold signals of oposite phase at each end of its drivers).
Both the active circuit and the passive transformer solutions have their own benefits and drawbacks. They both add distortion, and they add complexity.
Here's an alternative, quick and easy way to add switchable balanced/unbalanced inputs to a 4-ch balanced β22 amp. It adds just a 4PDT switch (or two DPDTs) and some wiring. No extra circuitry or transformer necessary.
This scheme could also be used on almost any 4-ch balanced amps.
Note that when switched to unbalanced inputs, the - amps have their inputs grounded, so that they become active ground buffers similar to a 3-channel active-ground amp (except in this case there is a separate ground amp per channel). In this mode, the "balanced" headphones are actually driven with unbalanced signals, only the + terminals of the headphone transducers "see" a varying signal. The - terminal is held to zero volts.

Hi Ti,

Does it mean that we should use an unbalanced headphone in this case and could use RCA output jack as pre-amp to drive unbalanced amp, with separate Zobel RCs? Thanks.

[Edited by housing on 11-06-2008 at 11:14 AM.]

amb

Headphone Council

Joined: Apr. 1, 2004
Locale: Sunnyvale, CA. USA
Total Posts: 4408

Message [#122] posted on: 11-06-2008 12:08 PM CST (US).

[Quoting housing]
Does it mean that we should use an unbalanced headphone in this case

You can use unbalanced headphones (i.e., standard TRS plug and jack) if you wire the jack's "ground" (sleeve) contact to PSU ground. You can also use balanced headphones with the L+, L-, R+ and R- connected as they should (to the respective amp outputs).

[Quote]
could use RCA output jack as pre-amp to drive unbalanced amp

The RCA output jack grounds should be tied to signal ground (i.e., input ground).

[Quote]
with separate Zobel RCs?

You shouldn't put in more than one zobel network per stereo channel. I suggest installing the zobel across the L+/L- outputs (as well as R+/R-).

housing

HeadWizer

Joined: Dec. 16, 2006
Locale: N/A
Total Posts: 163

Message [#123] posted on: 11-06-2008 12:31 PM CST (US).

Thanks, Ti.

rille

Member

Joined: May 22, 2007
Locale: Germany
Total Posts: 83

Message [#124] posted on: 11-21-2008 06:25 PM CST (US).

My new headphones (JVC HP-DX 1000) are quite sensitive and I can hear a slightly hum from my beta22 <frown>

The transformer is in one case with the amplifier (see picasaweb.googl...886596799703730). So I want to try another transformer with magnetic shielding. The transformer is availible with 65VA and the next one has 160VA. For a 3 channel beta22 80VA are recommended. I want to use 2x24V. With 200mA per board it will need around 41W (~68V after rectifier * 0,6A). So the 65VA transformer should be sufficient? Or should I buy the 160VA transformer? I'm afraid it would be too big for my my case. I could also reduce the bias a little bit.

The gain of 2 is still too high for my source (Terratec Phase X24 with 2Vrms). I will try a voltage devider because a gain lower than 2 is not recommended. It would be nice if a gain of 1 would be possible.

[Edited by rille on 11-21-2008 at 06:27 PM.]

amb

Headphone Council

Joined: Apr. 1, 2004
Locale: Sunnyvale, CA. USA
Total Posts: 4408

Message [#125] posted on: 11-21-2008 08:02 PM CST (US).

rille, 65VA is acceptable if the amp will be for headphones only. Perhaps you could turn down the bias a little (to 100-120mA through the output stage).

I don't recommend unity gain for the β22 because it would require very large compensation caps which would affect the bandwidth and speed. Use a voltage divider at the input if a gain of 2 is still too high.

housing

HeadWizer

Joined: Dec. 16, 2006
Locale: N/A
Total Posts: 163

Message [#126] posted on: 11-23-2008 02:10 AM CST (US).

Hi Ti,

For Gain=5x, should the value of C1 remains to be 33pF or should it be changed to 47pF also?

amb

Headphone Council

Joined: Apr. 1, 2004
Locale: Sunnyvale, CA. USA
Total Posts: 4408

Message [#127] posted on: 11-23-2008 03:54 AM CST (US).

[Quoting housing]
For Gain=5x, should the value of C1 remains to be 33pF or should it be changed to 47pF also?

C1 stays at 33pF.

housing

HeadWizer

Joined: Dec. 16, 2006
Locale: N/A
Total Posts: 163

Message [#128] posted on: 11-23-2008 07:58 AM CST (US).

[Quoting amb]
[Quoting housing]
For Gain=5x, should the value of C1 remains to be 33pF or should it be changed to 47pF also?

C1 stays at 33pF.

Thanks, Ti. <smile>

ferds

Member

Joined: May 21, 2005
Locale: philippines
Total Posts: 15

Message [#129] posted on: 11-24-2008 11:47 AM CST (US).

i was browsing HifiDiy.net forum today and found this b22 <shocked>

http://bbs.hifid...&extra=page%3D2
http://bbs.hifid...page%3D1&page=1

more here:http://s75.photobucket.com/albums/i297/ferds_calma/hifidiyB22

to moderator: please fell free to delete this post if its not allowed..

housing

HeadWizer

Joined: Dec. 16, 2006
Locale: N/A
Total Posts: 163

Message [#130] posted on: 11-24-2008 12:05 PM CST (US).

This is absurd!!! <shocked>

MrMajestic2

Member

Joined: Jun. 5, 2007
Locale: N/A
Total Posts: 90

Message [#131] posted on: 11-24-2008 12:19 PM CST (US).

It wouldn't be the first time AMBs design has been cloned. The owner of the photobucket album had cloned Mini3s for sale before.

cobaltmute

HeadWizer

Joined: Jul. 10, 2008
Locale: Toronto
Total Posts: 138

Message [#132] posted on: 11-24-2008 12:44 PM CST (US).

[Quoting MrMajestic2]
It wouldn't be the first time AMBs design has been cloned. The owner of the photobucket album had cloned Mini3s for sale before.

If you check that photbucket, he also did the PPA.

I was amused by the spray paint mask on some of the boards though.

housing

HeadWizer

Joined: Dec. 16, 2006
Locale: N/A
Total Posts: 163

Message [#133] posted on: 11-24-2008 01:07 PM CST (US).

These cloned b22 photos are not Ferd's. He just downloaded them from http://bbs.hifidiy.net/ and kept them in his photobucket. Don't misunderstand him! <evil grin>

ferds

Member

Joined: May 21, 2005
Locale: philippines
Total Posts: 15

Message [#134] posted on: 11-24-2008 07:30 PM CST (US).

those are not my boards, i only found them on hifidiy.net forum as housing have said..

I only uploaded them on my photobucket cos if you go to the hifidiy forums links, you wont see the photos..

[Edited by ferds on 11-24-2008 at 07:36 PM.]

lcp

Member

Joined: Nov. 12, 2006
Locale: N/A
Total Posts: 33

Message [#135] posted on: 11-29-2008 10:13 PM CST (US).

Amb,

I have got other problem when put all component(PCB, power supply) into case. The DC offset varied with volume pot. I think it is ground issue but I was exhausted to fix it. I have followed option3 at the wiring and ground page with 2 B22 board.

amb

Headphone Council

Joined: Apr. 1, 2004
Locale: Sunnyvale, CA. USA
Total Posts: 4408

Message [#136] posted on: 11-30-2008 05:59 AM CST (US).

lcp, due to its JFET inputs, the DC offset of the β22 shouldn't vary significantly with the volume control, unless you have a source connected and it has DC at its output.

lcp

Member

Joined: Nov. 12, 2006
Locale: N/A
Total Posts: 33

Message [#137] posted on: 11-30-2008 07:21 PM CST (US).

Amb,

I have usually checked the DC offset with source connected in my practice. So, I have to check DC offset with source disconnected?

Other question,
I have other B22 with problem(I have planned to do a balanced configure before).

the voltage measured at R9 is fine(can adjust to 4.5V). But, there was no voltage at R34/35 and heatsink is cold all the time. I have measured the Mosfet and they seemed OK(not shorted)

amb

Headphone Council

Joined: Apr. 1, 2004
Locale: Sunnyvale, CA. USA
Total Posts: 4408

Message [#138] posted on: 12-01-2008 01:07 AM CST (US).

[Quoting lcp]
I have usually checked the DC offset with source connected in my practice. So, I have to check DC offset with source disconnected?

You don't have to check it with the source disconnected, but the instructions I gave was to check the DC offset with the amp's volume control turned all the way down, so that you get a true sense of what the amp's own DC offset is. As soon as you start turning up the volume knob, if there is any DC from the source, it will be amplifed and what you're measuring would no longer be the offset of the amp itself.

Since this is a true DC amp, you should make sure your source doesn't have excessive output DC offset (should be less than a few mV). Otherwise, you should add output coupling caps to the source.

[Quote]
the voltage measured at R9 is fine(can adjust to 4.5V). But, there was no voltage at R34/35 and heatsink is cold all the time. I have measured the Mosfet and they seemed OK(not shorted)

Adjusting VR2 has no effect?
What are the voltages across R14 through R19?
Did you install the correct value trimpots at VR2 and VR3?

lcp

Member

Joined: Nov. 12, 2006
Locale: N/A
Total Posts: 33

Message [#139] posted on: 12-01-2008 02:30 AM CST (US).

The trim port is corrected.

The voltage are
R14 - 3.41V
R15 - 3.52V
R16 - 0.152V
R17 - 0.119V
R18 - 0.167V
R19 - 0.111V

amb

Headphone Council

Joined: Apr. 1, 2004
Locale: Sunnyvale, CA. USA
Total Posts: 4408

Message [#140] posted on: 12-01-2008 07:12 AM CST (US).

lcp, the voltages you listed look ok.

Note that the the trimpots are positioned as VR1, VR3 and VR2 from the input side to the heatsink side. Make sure you didn't reverse the positions of VR2 and VR3. Measure the resistance across their outer pins to verify.

If the trimpots are ok, then check your R34 and R35 resistors. Measure them to make sure they are still 0.47 ohms (most DMMs are not very accurate with such low resistances, but you should at least be able determine the resistance to within 1 ohm.

If R34 and R35 are ok, then check to make sure you didn't forget to install the wire jumpers at R30 and R31. Also verify that the resistances of R32 and R33 are correct.

[Edited by amb on 12-01-2008 at 07:18 AM.]

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