2. The beta 22 is silent near the lowest and highest volumes, but there is a hiss through the middle range of the volume control. Might the pot be the source of this hiss?

Thanks!!!

[Edited by jeremy.ggg on 08-12-2008 at 05:12 AM.]

1. What do you think the best power supply would be for the Monica?

2. Would it be a good idea to use the power from the σ22? If so, how might I accomplish it?

There appears to be a good deal of performance variation based upon the power supply for the Monica. The Monica seems to accept a wide range of input voltage, at least 12VDC - 24VDC.

This is the Monica:
http://diyparadise.com/usbmonica.html

This is a remark on the Monica power supply:
http://diyparad...id=77&Itemid=26

Thank you very much!

[Edited by jeremy.ggg on 08-12-2008 at 05:10 AM.]

That said, I prefer to keep the DAC and the amp in separate cases for flexibility and upgradability. In the future should you wish to upgrade to a better DAC, you don't have to take your amp apart.

Just for clarification the Monica requires a single rail 12 to 18VDC power supply.

Two more points about that - 1) as a former Monica owner and user I would recommend a different source for the beta22. The Monica is nice, but IMO not up to the level of performance of the beta22. 2) if you insist on using it I'd follow Ti's advice and house it in another enclosure for a whole host of reasons not the least of which is getting it away from any and all transformers so it doesn't pick up noise.

[Edited by izzyman on 08-17-2008 at 11:11 PM.]

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[Quoting audio nut]

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[Quoting amb]

audio nut, it would appear that the voltage gain of the amp(s) are two high for these particular Denon headphones. The default gain of β22 is 8 and CK²III is 9, which are good for a large number of headphones, but your headphones are probably too sensitive. Lowering the gain to 5 or even 2 should take care of the hiss. See the Parts list section of both amps' websites for info about what parts to change.

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Amb, Thanks, I will order out parts and try 5 gain, which looks like changing R3 to 392 ohm, R4 to 1.5K, and C2-C5 to 47pF, correct? Also, at 5 gain setting, I assume that you leave idle current at 160mA? Lastly, I also have a pair of 300ohm Senn HD-650's that I listen to, but not very often. No problem driving these with B22 set at gain 5, right? Thanks for your help!

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Just a quick update on the hiss problem: SOLVED! Got my parts in last week to take my B22 from 8 to 5 gain. This definitely cured 99% of the hiss between songs. My Denon AH-D5000's sound fantastic and for those who have low impedance headphones, you may want to build your B22 from the get go at 5 gain. Will save some time. Also, for those who haven't heard the Denon's, give them a listen if you get a chance. They sound great with this amp and are the best I've heard to date for dynamic headphones. Thanks again, AMB, for your help and support!! Tommy

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[Quoting amb]

Audio Nut, congratulations. Why not post some pics of your new β22?

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AMB, I guess I should, but it will probably never be cased up, as it's new home is in our master bedroom closet on top shelf along with the rest of my other headphone gear (preamp, component CD, etc..). Might bore some guys just seeing the populated PC boards with no enclosure! Another quick question. Do you need to go back in and check/adjust quiescent current after the amp has burned in for a particular # of hours? I set mine up originally to around 65 to 68mV. Not sure how many mA that corresponds to as I don't remember the formula. I felt the heatsinks, especially the outer 2, were gettting a little too hot.

[Edited by Audio Nut on 08-22-2008 at 08:48 AM.]

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[Quoting amb]

izzyman, if that tweeter has reasonably high efficiency and your room is not huge, then the β22 should be a great amp for it. I assume you will be using an active electronic crossover before the amp, which will remove the tweeter amp from bearing any burden of amplifying the bass frequencies. The resistive nature of the tweeter impedance also helps to make it easier on the amp. Nevertheless, because this ribbon is large and could be crossed over at the low midrange, to ensure that the amp's MOSFETs do not overheat, you should use larger, offboard heatsinks as shown in the "board and heatsinks" section of the β22 website.

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[Quoting amb]

Hard to say... most of the time it's due to accidents like the meter probes shorting from R9 to the ground plane, or some other mishaps. It's not likely that they would just blow for no reason.

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I just completed the electronic assembly of a three-channel β22, shorting R9 to ground during setup was the only problem I encountered during the build. Thankfully, it was the ground channel, so I was able to proceed with testing and auditioning the rest of the amp. Perhaps a future revision of the PCB could include test point pads for the setup measurements?

Unfortunately, my source exhibits 4.5 mV DC offset. Since I currently intend to use the amplifier solely with this source, I've simply used the β22's offset trimmers to null the DC. Any recommendations for a DC offset correction servo circuit, or should I just go with DC blocking capacitors?

So far, the results are spectacular! Thanks again, AMB, for sharing this quality project with the D.I.Y. community!

As for test points on the PCB, it probably won't happen but builders should invest in a set of DMM meter probes with mini-grabber clip ends. These will prove to be useful for all kinds of other projects as well, and in some cases critical for safety (such as in high voltage circuits). See the following article:
http://www.tubelab.com/MeterUse.htm

As for DC servo circuit, that is also unlikely to be added. Normally a DC servo is used as a self-corrective mechanism to eliminate the offset adjustment phase of the build, and to compensate for offset drift due to thermal changes. It is not really intended to null externally-induced offset (such as from the source). The β22 should experience negligible thermal offset drift.

[Edited by amb on 08-25-2008 at 12:05 PM.]

I used to have oscilloscope probes with the mini-grabber clip ends, back when I had a fully equipped lab. I suppose I ought to invest in a pair for the DMM. I naively assumed the solder resist mask would prevent shorting to the ground plane.

[edit] One replacement Q25, and all is well with my ground channel module. [/edit]

[Edited by helzerr on 08-28-2008 at 06:41 PM.]

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[Quoting Sid]

What is the recommended wiring to use for a 3-board headphone amplifier (using an E22 backplane) that should also function as a pre-amp?

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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.

[Edited by amb on 08-29-2008 at 12:31 PM.]

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[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.

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You might want to include this on your B22 website.

[Edited by set300b on 08-29-2008 at 05:01 PM.]

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[Quote]

You might want to include this on your B22 website.

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