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DIY Workshop » β22: A discrete, cascoded, fully complementary, pure class A amplifier (part 3) amb  Headphone Council Joined: Apr. 1, 2004 |  Message [#1] posted on: 07-30-2008 02:35 AM CST (US).   |  |  This is a continuation of the β22 amplifier thread. A new thread is needed here because of the 1000-post limit per thread on this forum. Links to previous parts: Let's keep a good thing going... - Official β22 amplifier website [Edited by amb on 07-30-2008 at 02:35 AM.] |
| fc911c Member Joined: Jun. 6, 2008 | Message [#2] posted on: 07-31-2008 12:05 PM CST (US). | | Hi I have been listening to my B22/E22 for over a month and suddenly today the left channel dropped out. I am getting power to the board and both led's are light. I am not getting 4.5v to R9, 10, 11, and 12. thanks |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#3] posted on: 07-31-2008 01:24 PM CST (US). | | fc911c, check the voltage across the C-E junctions of Q25 and Q26. If either are more than a couple of volts then that transistor is blown. |
| fc911c Member Joined: Jun. 6, 2008 | Message [#4] posted on: 07-31-2008 02:39 PM CST (US). | | Thanks that did it, I am up and running again. What might have caused them to blow? Thanks |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#5] posted on: 07-31-2008 05:38 PM CST (US). | |
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. |
| Audio Nut Member Joined: Feb. 12, 2008 | Message [#6] posted on: 08-06-2008 03:18 PM CST (US). | | Gentlemen, I am about to start the initial setup of my 3 channel B22. Can you just short the input pads to do setup, or do you have to wire up the volume pot for all 3 boards and set to minimum? I know you you can do this on the CK2III, which I built, but wasn't sure if it's ok to do this on the B22. Since the ground channel doesn't use R1, not sure if shorting the input pad on this channel would work. Thanks. |
Dougigs HeadWize Fanatic Joined: Sep. 21, 2005 | Message [#7] posted on: 08-06-2008 04:35 PM CST (US). | | Yes, you can short the inputs to ground. The ground channel's input is already connected to ground! |
| Audio Nut Member Joined: Feb. 12, 2008 | Message [#8] posted on: 08-06-2008 04:50 PM CST (US). | |
Dougigs, Thanks for clarifying! |
n_maher HeadWize Fanatic Joined: Dec. 15, 2004 | Message [#9] posted on: 08-08-2008 11:39 AM CST (US). | | I've been meaning to post a final follow up to my transformer research and kept forgetting, I blame a distinct lack of sleep! Anyway, as previously mentioned the first shielded transformer that I purchased from SUMR was not sufficient to completely quell the buzzing that I was hearing when using IEMs with the beta22 that I'd built. Therefore I asked Richard what it would cost to get a fully shielded and potted transformer from him, below is his response.
He also noted that the final size of the trafo would be approximately 4-1/8" in diameter and 2-1/16" high. The transformer leads can be made to exit either on the bottom or the side of the enclosure, at the buyer's option. And the part number listed above for the transformer is for a 120V primary, dual 30V secondary, 80VA transformer. Here's a picture of what the finished product looks like:
Given that the standard Avel trafo will run around $40 (shipped) and has been known to cause hum issues I would seriously consider using one of these fully shielded units since it would mean that single box, closely packed layouts could be used. That would save the hassle of a separate PS enclosure and the resultant umbilical cord and connectors. I may in fact try this on my next beta build. I hope that this information is help to other builders. |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#10] posted on: 08-08-2008 01:16 PM CST (US). | | n_maher, so the other SumR transformer you tested wasn't fully shielded? |
| audio nut Member Joined: Feb. 12, 2008 | Message [#11] posted on: 08-08-2008 08:19 PM CST (US). | | Guys. I got my 3 channel B22 up and running this evening. I've listened to it for an hour and a half and have a few questions. With my main headphones, which are the very low impedance (25 ohm) Denon AH-D5000's, I'm getting what I call a very faint amplifier hiss between songs. It's not totally "black" or silent with nothing playing. It's nothing I can't live with, but was wondering if there's anything I can do to eliminate it? I might also add that my CK2III that I have has the same very slight hiss between songs. Was wondering if setting the idle current to less than the 160Ma would have any effect on this as I know this amp was also designed to drive small speakers? Could it just be the fact that my headphones are such low impedance and super sensitive? Any ideas, suggestions or opinions greatly appreciated. Oh, by the way, the B22 sounds FANTASTIC and will post further listening comparisons between my CK2III and Bijou, all of which I've built over the last 5 months. Thanks! |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#12] posted on: 08-08-2008 09:42 PM CST (US). | | 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 CKČIII 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. |
| audio nut Member Joined: Feb. 12, 2008 | Message [#13] posted on: 08-08-2008 10:17 PM CST (US). | |
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! [Edited by audio nut on 08-08-2008 at 11:51 PM.] |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#14] posted on: 08-08-2008 11:38 PM CST (US). | | Yes, that's correct. The quiescent current is not affected. A gain of 5 is still sufficient for HD600s (but probably not for some 600 ohm Beyers or AKGs). |
set300b HeadWizer Joined: Aug. 1, 2001 | Message [#15] posted on: 08-09-2008 08:19 PM CST (US). | | AMB, what values should C2-C5 and R3-R4 be for 4x gain? I am contemplating a six-board version for the total gain of 8x to drive the 600 Ohm Beyer 880's. I hope 8x gain is enough. |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#16] posted on: 08-09-2008 09:42 PM CST (US). | | set300b, you can use R4 = 1.54K, R3 = 499R, C2-C5 = 68pF. That said, the difference is less than 2dB between 8x gain and 10x gain. |
| n_maher HeadWize Fanatic Joined: Dec. 15, 2004 | Message [#17] posted on: 08-11-2008 09:47 AM CST (US). | | Correct. Richard installed a layer of core banding around the circumference of the transformer just below the final wrap of mylar so it did not have shielding on the top or bottom of the unit which is where I suspect the problematic stray field was coming from (as I previously experienced). |
| tenorsaxophone Member Joined: Feb. 27, 2008 | Message [#18] posted on: 08-11-2008 02:43 PM CST (US). | | Hello! I have just a 100KΩ potentiomenter. Is it good for Beta22 amplifier instead to use a 50KΩ? Many thanks. Best regards. Davide. [Edited by tenorsaxophone on 08-11-2008 at 02:47 PM.] |
| tenorsaxophone Member Joined: Feb. 27, 2008 | Message [#19] posted on: 08-11-2008 02:44 PM CST (US). | |
So I could save money of a new potentiometer.... [Edited by tenorsaxophone on 08-11-2008 at 02:50 PM.] |
| n_maher HeadWize Fanatic Joined: Dec. 15, 2004 | Message [#20] posted on: 08-11-2008 03:07 PM CST (US). | |
I've used a 100k pot in a beta22 and could not hear any difference from the one that I built using a 10k pot. |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#21] posted on: 08-11-2008 11:26 PM CST (US). | | The higher the source impedance (which includes the pot value at low settings), the higher the likelihood of noise, because a pot is a resistor, and some types of resistor noise is proportional to resistance (google "resistor noise" if you care). The choice of pot resistance is constrained by noise at one end, vs. loading the driving source excessively at the other end. I specified 10K to 50K ohms because that is the "sweet spot" range. However, in practice 100K works out fine in many builds, as long as the amp's gain isn't excessively high. |
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