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Planning a Beta 22--5 board setup?

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Planning a Beta 22--5 board setup?

Postby dpitch40 » March 4th, 2013, 6:47 am

I am in the planning stages of my Beta 22 amp. I am planning to use it largely to drive a pair of HD800s, so trying to maximize transparency. To this end I was going to build a 4-board amp and convert the HD800s to balanced, but I'd also like to maximize quality for my single-ended headphones so I was thinking of adding an active ground to one of the unbalanced circuits for a 5-board setup. Basically it would be like a cross between the recommended 4 and 6-board setups for combining balanced and unbalanced circuits.
Image
I don't care about actively grounding both single-ended channels because, with my tastes in music, tandem listening sessions on this amp seem highly unlikely. Anyway, a few questions about this setup:

1. I have a feeling there is a reason I have not seen anyone else planning a 5-board build. What are the issues with this setup? I am especially thinking of the power supply; having one Sigma 22 driving three boards and one driving two seems imbalanced, unless I had one driving all four signal boards and a second driving just the active ground. Or would it be feasible to use just one Sigma 22 for everything?
2. Assuming I get two Sigma 22s, I am currently thinking of using two Hammond rack-mountable cases, one 17"x13"x3.5" and one 17"x8"x3.5" stacked on top of each other, but their construction is aluminum which I have heard is worse at blocking EMI. Should I look for a steel option, or would the distances involved in two separate cases be sufficient to prevent humming? (I would presumably put the transformer coils as far away from the Beta 22 boards as possible)
3. In general, how do people get their front and back panels customized with the right openings? Do you order a premade case and have the panels machined? Order custom-made panels? This is the biggest part of the build that I am unsure of as of now.
4. If anyone knows of a good, transparent balanced DAC to go with this, recommendations are welcome.
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Re: Planning a Beta 22--5 board setup?

Postby amb » March 4th, 2013, 1:49 pm

1. I have a feeling there is a reason I have not seen anyone else planning a 5-board build. What are the issues with this setup? I am especially thinking of the power supply; having one Sigma 22 driving three boards and one driving two seems imbalanced, unless I had one driving all four signal boards and a second driving just the active ground. Or would it be feasible to use just one Sigma 22 for everything?

Yes, the issue is that you cannot evenly divide 5 amp boards into 2 power supplies. The amp will "work" as you described. But the reason why this is an issue is not so much a matter of current draw imbalance. It's wordy to describe this so bear with me.

One of the key benefits of balanced operation as well as 3-channel active ground operation is supply rail current cancellation. In a balanced amp with push-pull output topology, when there is no signal, the quiescent current on each of the hot and cold amp outputs flow from the V+ rail to V- rail through the output transistors, with no current going through the headphone transducer.

When you turn up the volume to play music, as long as the amp remains operating in class A, in additional to the abovementioned quiescent current, when the signal voltage swings in the positive direction, there is now current flowing from the hot amp's V+ through the top transistor into the transducer and then to the cold amp's bottom transistor into V-. Simultaneously, due to the differential nature of the output, current also flows from V- through the hot amp's bottom transistor into the transducer and then into the cold amp's top transistor into V+.

When the signal swings negative, the exact same thing happens except the current directions are all reversed. See illustration below. The dotted lines are when the amp is in no signal condition. The solid lines show when the amp is amplifying voltage:

pic2.png

In all cases the supply currents on the rails sum to zero, because the hot amp's current draw is exactly out of phase of the cold amp's. This results in completely quiescent supply rails at any output level (as long as the amp stays in class A operation). Also since none of the transducer's output current go to ground, the ground is also completely quiescent.

For a 3-channel active-ground configuration, the situation is similar, except that the ground amp handles the signal currents for both stereo channels. The total rail current still sums to zero, and the ground is also quiescent.

pic4.png

To maintain these benefits, there are some requirements on the power supply topology. In a balanced amp, you could put all four channels on the same supply, or have one supply for each of the left and right channels (two boards each), and the rail cancellation will work. In a active-ground 3-channel amp, you need to have all three amp boards running off the same rails to get that benefit.

Since you're sharing the balanced left hot and right hot amps with the unbalanced configuration, you would only feasibly get rail cancellation if all five amp boards run off the same power supply. The σ22 is designed to handle up to four β22 boards with the stock onboard heatsinks, and has four sets of output terminals. If you use one σ22 to run five boards, you'll need to use bigger heatsinks and provide plenty of ventilation. You may also need to reduce the bias settings on the β22 boards to lower their current draw. Depending on the impedance of the headphones you use, reducing the bias too much may cause the amp to go into class AB operation on signal peaks.

Adding a passive ground unbalanced output to the mix further complicates matters. To contrast with the above scenarios, let's complete the picture by seeing what happens in a conventional passive-ground output topology:

pic1.png

As you can see, when amplifying signal, during positive signal swings the current flows from V+ through the top output transistor into the transducer and then into the positive rail capacitor and power supply ground. When the signal swings negative, current flows from ground and negative rail capacitor into the transducer, and then through the bottom transistors into V-. There is no rail current cancellation, and there are currents flowing in and out of ground. Since ground is comprised of real-world wires/PCB traces, and add to that the rail capacitors' ESR, you have a non-zero ground resistance as seen by the amp. According to Ohm's Law, when there is current flowing through a non-zero resistance, there will be non-zero voltage. In other words, the ground is not always at 0V, but wiggling slightly above and below 0V with the signal.

2. Assuming I get two Sigma 22s, I am currently thinking of using two Hammond rack-mountable cases, one 17"x13"x3.5" and one 17"x8"x3.5" stacked on top of each other, but their construction is aluminum which I have heard is worse at blocking EMI. Should I look for a steel option, or would the distances involved in two separate cases be sufficient to prevent humming? (I would presumably put the transformer coils as far away from the Beta 22 boards as possible)

Transformers emit magnetic fields and cause hum, the farther you put the transformer from the amp boards (and input wiring and volume pot/attenuator), the better. Just using steel cases may not be enough unless all panels are steel and without vents, and steel is hard to work with (drilling, etc).

3. In general, how do people get their front and back panels customized with the right openings? Do you order a premade case and have the panels machined? Order custom-made panels? This is the biggest part of the build that I am unsure of as of now.

Some people DIY the entire case, others buy pre-made cases and do drilling/machining of the panels on their own, and then there are also outfits like Front Panel Express that would machine very professional-looking panels for you, either on your own material or theirs.
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Re: Planning a Beta 22--5 board setup?

Postby joeyjojo » March 4th, 2013, 4:22 pm

dpitch40 wrote:1. I have a feeling there is a reason I have not seen anyone else planning a 5-board build. What are the issues with this setup? I am especially thinking of the power supply; having one Sigma 22 driving three boards and one driving two seems imbalanced, unless I had one driving all four signal boards and a second driving just the active ground. Or would it be feasible to use just one Sigma 22 for everything?

4. If anyone knows of a good, transparent balanced DAC to go with this, recommendations are welcome.


1. Cost and size. I started planning over a year ago for a 6 board setup and after soldering the first two I quickly realised it was a huge waste of money and time. Personally I wouldn't build a 4 board amp unless I intended to use speakers frequently as 2 or 3 boards are plenty good enough for headphones/preamp, with the added bonus of not needing to do any recabling. The AMB "reference" amp is 3 channel as a compromise. Don't believe the hype around "balanced" headphones. The electrical advantages are very real of course as explained above, but these will only give you a sense of satisfaction rather than a sonic improvement (speculation!).

4. AMB's very own gamma2 of course! 8-)
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Re: Planning a Beta 22--5 board setup?

Postby amb » March 4th, 2013, 5:07 pm

The γ2 does not have balanced outputs.

I am working on a DAC that will have that and more, but it won't happen in the near term.
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Re: Planning a Beta 22--5 board setup?

Postby dpitch40 » March 4th, 2013, 5:39 pm

Okay...I understood some of that. Leaning more towards a 3 or 4-board setup now. Could you explain more about the difference between balanced and 3-board active-ground setups? Can anyone who has heard both attest to this?

Does distance matter for EMI with a shielded transformer?

Also, if hypothetically I made a 6-board setup with two Sigma 22s each running three boards, would supply rail cancellation still be an issue?
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Re: Planning a Beta 22--5 board setup?

Postby amb » March 4th, 2013, 10:40 pm

dpitch40 wrote:Okay...I understood some of that. Leaning more towards a 3 or 4-board setup now. Could you explain more about the difference between balanced and 3-board active-ground setups?

3-channel active ground configuration allows you to use unbalanced sources and "normal" headphones (i.e., with TRS plug, no retermination/recabling needed) and get most of the benefit of a fully-balanced configuration. If you have balanced sources then it makes sense to build the 4-channel balanced configuation so that it stays balanced all the way to the headphones.

Does distance matter for EMI with a shielded transformer?

Yes, since shielding merely reduces the magnetic leakage, not totally elminate it. But the weaker the magnetic fields, the distance required for hum-free operation is also reduced.

Also, if hypothetically I made a 6-board setup with two Sigma 22s each running three boards, would supply rail cancellation still be an issue?

If you're talking about a mixed balanced and unbalanced configuration, then which σ22 would you connect each of the two active ground boards to? Think about it based on my answer in my previous post, and the answer should become obvious.
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Re: Planning a Beta 22--5 board setup?

Postby joeyjojo » March 5th, 2013, 1:37 am

amb wrote:The γ2 does not have balanced outputs.


Whoops, missed the b word.

dpitch40 wrote:Okay...I understood some of that. Leaning more towards a 3 or 4-board setup now. Could you explain more about the difference between balanced and 3-board active-ground setups? Can anyone who has heard both attest to this?

Does distance matter for EMI with a shielded transformer?

Also, if hypothetically I made a 6-board setup with two Sigma 22s each running three boards, would supply rail cancellation still be an issue?


These are all questions that have come up time and time again if you search back through the forum, the head-fi thread, and (if it was still online) the headwize thread. When I was in your position last year I ended up being convinced by linuxworks that anything more than a conventional passive ground (2 board) topology was overkill, considering how "overkill" the amp is by design!

Starting from there, if you then go to 3 boards you get a bit more symmetry as the current is sunk to the 3rd board taking the PSU out of it (although the s22 is very robust and more than capable of this duty) but for +50% cost and space, although you may still use normal headphone cables. The backplane makes this very tidy. It's the "recommended" config for these reasons. Then 4 boards gives you the really nice (in theory!) current cancellation described above and twice (almost) the voltage swing which is a huge pro for speakers, but at +100% cost and space AND you will need to reterminate most headphones to 4-wire connectors.

To go to 5 or 6 is honestly not worth considering due to the huge cost and space requirements, although I spent several months trying to plan it out!

Distance is the only way to keep the sensitive boards away, which is why almost all build are in 2 boxes.
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Re: Planning a Beta 22--5 board setup?

Postby dpitch40 » March 5th, 2013, 4:47 am

Okay, think I may go for a simple 3-board. I only have to worry about distance between the transformer and Beta 22 boards, not the Sigma 22, right?
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Re: Planning a Beta 22--5 board setup?

Postby dpitch40 » March 5th, 2013, 9:16 am

I'm tentatively making a parts list for a 3-board amp. Question on the parts list: If I'm using the default gain of 8x, would I make capacitors 1-5 on the main boards all 33 pF, and capacitor 1 on the ground board 33 pF and 2-5 on the ground board 100 pF? In other words, is capacitor 1 always 33 pF no matter the type of board or gain? The note seems to be for all five of them, but only specifies values for 2-5.

Also: I am having some trouble selecting the display LEDs for the boards. Do I have to worry about the forward voltage over these LEDs or will most of the voltage drop be through the resistor? What form factor do I want? (Digikey has very few through hole high brightness LEDs and they either are non-stock or have three pins)

Could anyone direct me to a resource for choosing a gain value? ("Gain" is too common to search the forum for it)
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Re: Planning a Beta 22--5 board setup?

Postby amb » March 5th, 2013, 1:10 pm

dpitch40 wrote:Okay, think I may go for a simple 3-board. I only have to worry about distance between the transformer and Beta 22 boards, not the Sigma 22, right?

Yes, for the most part. In some power supplies the rectifier diodes may also emit interference, but not nearly as bad as what a transformer would do. The fast recovery diodes used on the σ22 seem fairly good in this regard, but if you will be putting the σ22 in the same case as the β22s, then try to put some distance between them, and orient the σ22 such that the rectifiers are on the far end.

I'm tentatively making a parts list for a 3-board amp. Question on the parts list: If I'm using the default gain of 8x, would I make capacitors 1-5 on the main boards all 33 pF, and capacitor 1 on the ground board 33 pF and 2-5 on the ground board 100 pF? In other words, is capacitor 1 always 33 pF no matter the type of board or gain? The note seems to be for all five of them, but only specifies values for 2-5.

Correct, C1 is 33pF in all cases. C2-C5 must change with the gain setting.

Also: I am having some trouble selecting the display LEDs for the boards. Do I have to worry about the forward voltage over these LEDs or will most of the voltage drop be through the resistor? What form factor do I want? (Digikey has very few through hole high brightness LEDs and they either are non-stock or have three pins)

Most of the voltage drop will be through the resistor. I recommend T-1 (3mm) LEDs.
Try Vishay TLHR4400, TLHR4401 (red) or TLHB4400, TLHB4401 (blue), or other colors in these series. Use the default 10K resistor value with these and you'll get good results.

Could anyone direct me to a resource for choosing a gain value? ("Gain" is too common to search the forum for it)

You choose the gain based on the output level of your source(s) and the sensitivity/impedance of your headphones.
For a source like the γ2 DAC (with 1.4Vrms output at 0dBFS), a gain of 2-5 is good for 32-150 ohm low impedance headphones, and 5-8 for high impedance headphones up to 300 ohms, and more gain for 600 ohm headphones.
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