mwl168

I am thinking about building this amp. Is V1.43 that uses MJE15030/15031 the latest board? Anyone has boards for sale? I have 2SJ109/2SK389 on hand so I may be able to build older versions that use JFET input too.

Quick update on the status of the GB - we are in the process of testing the boards before sending them to the fab house for estimates. Will provide another update as progress is made.

You can try do a simple diode test of those transistors and compare the results between the two boards to see if one or more are out of ordinary. Make sure the capacitors are completely drained before you perform the tests though.

I would try replacing the correct value resistor first and test the board again before swapping out the LT1021.

My Blue Hawaii without servo takes about 90 minutes for the offset/balance to stabilize. Although after 60 minutes or so they do not drift more than a few volts. I adjust offset/balance after two hours running and leave them be from there on. From power on to power on these voltage will vary a bit but usually within a volt or two once stabilized. There shouldn't be need to readjust on power-on unless the ambient temperature changes a lot or new tubes are inserted. Does the balance, measured between L+ and L-, R+ and R-, also varies significant from power-on to power-on?

It was not clear to me reading your posts if the -27V is the DC offset from one phase of the outputs and if it occurs to both channels? If it only occurs with one channel you can rule out if the tubes are the culprit or not by swapping the tubes around.

Table updated.

It’s better you clearly list which boards and quantities you need.

Do you need the mini T2 amp boards only or do you also need the PS boards?

Nicely done JoaMat! Taking a blind shot in the dark, if the hum only occurs in one channel and goes away after a few minutes, could it be 9ne of the tubes?

Table updated.

The SS Dynalo is really intended for both balanced input and output operations. The original Dynalo would have been a better choice for single end operation but I have not seen original Dynalo board being offered for many years. If you have not already done so you can read up the current feed back amp (CFA) also designed by Kevin which would be a good option for single end operation. As for PS, the Sigma22 is a solid choice. another option is Kevin's Golden Reference (GRLV) which many here has used to power their SS Dynalo, myself included.

Table updated.

Have been having some offline discussions with JoaMat and Jose regarding the possibility of trimming the offset if desired. JoaMat came up with the idea of trimming the 100R resistors in series between the KSC2752 emitters and B-. He modeled it in a LT Spice model and I decided to give it a try. With my amp running at 14mA bias current, without this mod all the DC offset was between + 9VDC to + 4 VDC and balance was under 6VDC and I consider it totally fine. This exercise was more experimental and to satisfy the OCD part of me. I put a 50R resistor and 100R trimmer in series and adjust the sum to 90R based on JoMat's calculation. It can certainly be done with a single 150R trimmer, a fixed resistor in series with a trimmer is to protect against the small odd should a trimmer fails. I stopped making further adjustment once I got all the offset within 0.5VDC and balance within 0.2VDC. A few things to observe: 1. Turning one phase DC offset lower raises the offset in the opposite phase slightly. This gets a little tricky trying to zero out offset of both phases at the same time. 2. The bias current raises slightly as I adjust down the offset. In my case, the resulting increase in bias current was about 0.7mA. This matches what JoaMat saw in his LT Spice model. 3. The final resistance on all 4 "combo trimmers" fall between 86R - 90R. Take this as a reference only as this will vary from amp to amp and dependent on the bias current setting. I listened to the amp both before and after adjustment and frankly I could not detect a difference.

Although I’ve always liked the sound of the Maggies I never had the pleasure of owning a pair. I had a pair of Apogee Duetta Signatures that I loved. I foolishly sold them to a friend because I could never set them up properly to get reasonable soundstage in the house I had. There is another major disadvantage of the Apogees - they need monstrous amps to drive them properly. The Maggies on the other hand seem much less peaky and open up many options for amping.

Modern day Tympani?

You can find the information in this thread. The main board has the rectifiers, filtering caps and timed delay circuit for the HV among other things.

Table updated.

I started a thread dedicated to the group buy of Kerry's PS boards and JoaMat's mini T2 boards. Let's move all GB related discussions to the new thread. Thanks!

I am moving Kerry's mini GRHV\GRLV GB discussion to its dedicated thread here. I also included JoaMat's mini T2 here to see if we have enough interest for a GB. The table will be updated periodically as more people participate. Apologize if I missed someone or something - just let me know so I can correct the data. I put down one set for those that have not decided on the number of sets they want. The plan is to have assembled GRHVxxx boards, rest of boards will be PCB only. See discussions on the PS in this thread. mini T2 build discussion in this thread. Construction notes: R3 and R2 determine the regulated voltage for GRLV. The formula is Vout = (R2 + R3)/R3 * Vref. Vref is the reference voltage of U4 (LT1021). R7, R8, R9, R10 determine the regulated voltage for GRHV. The formula is Vout = ((R8 +R9 + R10)/R7 + 1) * Vref. Vref is the reference voltage of IC1 (LT1021). You want to keep R8, R9 R10 values close so they have even voltage drop across each of them. On the PSU Main board, R22 (100K) and C17 (470uf) set up the delay time for HV. The 100k/470uf combination provides a roughly 45 seconds delay. Increase R22 or C17 to increase the delay time. C17 needs to be rated 5VDC or higher. Board dimensions: mini T2: 160mm x 100mm GRHVxxx: 41mm x 31mm GR78xx/GR79xx: 30mm x 20mm Main board (PSU): 140mm x 90mm See BOM for boards at the bottom. Blank in the GRHVxxx-bare column indicates that the participant has not responded on this thread to indicate the preference for bare or partially-assembled boards and is deemed to opt for partially-assembled board by default as previously communicated. BOM: GRHV78xxx-BOM.xlsx miniT2v.22_Date_2020-02-16.xls GR78xx - V2 .xlsx GRHVBase-BOM - V2 production.xlsx GR79xx-V2.xlsx

That's about the offset values I am seeing in my build too. I don't think it's a problem to worry about since they are below 15V. I consulted with Kevin a while ago about possibility of bringing down the offset and he think it would involve resistor trimming...

Happy to organize a GB if there is enough interest. Please raise your hands and state quantity desired here if interested.

Nice one Steve! Is that the Mercury or IVY I/V stage you are using? I also have the Cronus re-clocking and isolator with my dual-mono Buffalo II and , in my experience, a worthy investment!

I've discussed this topic with JoaMat early on. His advise is that the +220VDC be supplied by its own regulator instead of using a drop resistor from the B+. The top device, HN4A51J, is a dual transistor chip and need to be careful not to overheat it. The draw on the +220VDC rail should be around 20-30mA both channels combined.

See the zip file below; JoaMat kindly put together a list of component value changes for the mini T2 that the builders can apply to the V1 boards received from me. These values will put your mini T2 close to the operation points of the original DIY T2. Following the values on the silkscreen without changes will still result in an working amp. The zip file contains a complete list of the changes and pictures of the boards outlining the changes. I also attached a screen shot that shows the list of changes. Thanks very much JoaMat for doing this! Changes mini T2.zip