mwl168

I made some modifications to my Megatron. Most of them are under the hood. changed the CCS tubes filament supply transformers from 25VA Antek AN 0206 to 50VA AS 0506 (one for each channel). Now the under-load filament voltage is at 6.15v. changed the transformer supplying the front end filament and current sink to 50VA Antek 0512 to accommodate the higher filament supply when using E80CC in place of the 12AU7 and drill some holes on the PSU chassis to help with cooling. added an LED indicator on the amp chassis which comes on when HV supply is on.

It would also be helpful if you can be more specific about what you meant by the "distortion" you are hearing especially on that particular Youtube piece you provided as an example and where the distortion occur (as how many seconds into the piece...).

IIRC, we agreed that the tube unbal-bal boards would be 2mm thick to better handle the mounting/dismounting of tubes. I am fine with 2mm thickness.

Paid. Thanks!

No sure this applies to this situation, but are the inputs shorted to ground when you ran your tests and adjustments?

Thanks Jim!

Switched to matched quads of Svetlana EL34 for the CCS tubes and SED EL34 for the gain tubes and output balance and offset for both channels are all under 3VDC now.

Thanks Jim! That is very helpful! So I really need matched octets or at least two matched quad, one for each channel to get the lowest offset. I remember your past comments on filament voltage. What are your thoughts on my CCS tube filament supplies being just slightly over 5.7v? I have been wondering if I should look at options to bring them up.

Matched quad of EH 6CA7 for the output CCS and matched quad of SED EL34 for the output gain tubes. I don’t believe the CCS tubes need to be matched to the gain tubes. Maybe Kevin, Birgir or someone can correct me if I am wrong. Siemens 5814A for the 12AU7 position, Telefunken for the 12AX7. I was using Valvo ECC82 and Sovtek 12AX7 while I was doing early testing. Edit: the output tubes and the CCS tubes do need to be matched to obtain the lowest possible DC offset and balance. Mixing the 6CA7 and El34 was a bad idea.

Is it the original, single ended Dynalo or the SS Dynalo? And how much voltage drop between the pre-reg and regulated output?

Did some fine tuning on my Megatron. Bring the regulated HV down to 430VDC rails and changed one resistor from 21K to 15K to supply 300VDC to the front end tubes. In my case the front end tubes draw about 8.5mA total. Also changes the EL34 tubes around in an attempt to bring down the output offset. It's now down to 12VDC yet not as low as I would like. Have a few more sets of EL34 to swap and match to see how low I can bring the offset down to. The output balance is around 3VDC for both channels. The toggle switch on the back of the PSU controls the power to the CPC1117N relay on the GRHV and function as the HV delay switch.

Hi Jose: See my post in the Magatron thread - I used the KSA1156 (flipped around for the different pinout) based on advise from Kevin and it works fine. I believe STN9360 (not STN9630) should work as well.

Doing some initial testing of my Megatron. This is the latest version from the GB with the 12.6v filament for the front end tubes. It's making music but I do have some more work to do yet. The Antek AS-2T350 with its rated 350vac secondaries (sagged to 340vac under load) proven not enough to sustain the 450VDC targeted regulated voltage. The GRHV regulated rails sagged to about 433VDC under the load of the Megatron. As such, the intended +300VDC for the front end measured only +264VDC. Also, the 2 Antek AN-0206 transformers I used for the CCS EL34 filament supply only measured about 5.75vac, within the 10% tolerance but a bit low for my liking. May need to explore other options. These with the main ac measured about 117vac. The output balance measured 2.8VDC and 0.7VDC and the offset measured -15VDC and -17VDC (L/R). A few points worth mentioning: 1. I use KSA1156 in place of the 2SA1968 (need to rotate the parts to observe the different pinout) 2. I use a pair of 7812/7912 in the LV part of the GRHV to supply the front end tube filament and the current sink. I mounted them on a 60mm tall heatsink and it appears to work fine for the jobs intended. 3. I am using the split GRHV boards with the CPC1117N delay. I used the 12VDC (with a series resistor to limit the current) from the LV through a toggle switch to manually control the HV delay. It makes for a bit of messy wiring but works fine. Back to work to adjust the GRHV for 425VDC rails and tweak the resistors in series of the B+ to provide 300VDC rail for the front end tubes. EDIT: Forgot to mention perhaps the most important point - the front end tube filament supply connection: if you are using DC filament like I do, make sure you connect the ground wire of the filament supply to the ground pin (which is marked 12.6v on the board) and the 12.6VDC supply wire to the other connector which is NOT the one marked "12.6v". The one marked "12.6v" is actually connected to ground.

You can get a new HD650 for around $360 and HD600 for under $300 on Amazon. I have seen both at lower prices there from time to time too.

Have been using a dual-mono Buffalo II for close to two years now, used a stereo Buffalo II for many years before that, also built a Buffalo III for a friend. No experience with the Buffalo Pro and the yggy.

Ordered a USB - serial cable but have not received it after several weeks so have not tried configuring the filter files etc. using a Mac yet. Also have not tried the diamond buffer nor the unbal-bal with the DAM1021 yet. I built this DAC for my son to use at school with the single-ended Dynalo so really not intend to max it out. Although I will likely be exerimenting with it. I personally did not find the OP based balanced output as bad as some have made it out to be on the web.

With a pre-built LM317/337 based PSU, it took me less than 30 min. to get the first DAM1021 to start playing music with a SPDIF input and a digital volume wired. It plays both PCM and DSD. The sound is hard to beat for the money.

There is a new version KGBH that uses current parts. I ran a GB on it last year. In addition to replacing the 2SA1968 as the CCS parts as Pars pointed out, this version also replaces the 2SK216/2SJ79 JFET with current production BJT parts.

The last part finally came in so here is the finished build. I did run into a weird problem using the Cronus/Hermes/Amanero with this DAM1021 - the combo plays music through Audirvana fine but had serious dropout issues when playing YouTube and Amazon videos. I was able to find a few Amanero CPLD/CPU firmware combinations that seems to have addressed the issue. I started a thread over at DIYAUDIO on this topic. Right now I am using CPLD firmware "slave_for_1080" and CPU firmware "CPU_1099c". Even though the DAM1021 has its own isolator and re-clocker I feel subjectively adding the Cronus/Hermes is an improvement over using Amanero alone.

I vote the Megatron style.

Wonder if anyone is interested in the solid state ubal/bal and diamond buffer boards?

Just added my orders to the spreadsheet. I have one request; for the unbal/bal tube board, since we'll be mounting the tube sockets on it, should we consider ordering the boards in 2mm thickness?

@ Pars: I suspect the ones I got are poorly made copies of the ones you got from THL which is actually located in Taiwan. I guess the trick is which vendors can one trust to get the quality ones. @ Jose: I like Neutrik product very well and use them when I can. I this case it's just much less work to drill one hole per connector than 3.

A word of caution about the CMC 805-2.5F chassis-mount RCA sockets I recently bought and received from AliExpress (also widely available on eBay and other places). They don't work. I don't know if all these similarly-labelled RCA sockets are all made by the same manufacturer. The ones I received look well-made but are out of spec. The ones with red insert are way too loose and won't hold a tight connection and the ones with white insert are way too tight and need hercules force to insert (and good luck trying to unplug it afterwards). I bought 5 pairs and all of them have the same issue. It's a pity, they do appear well made with good quality materials otherwise. Hope others can chime in with their experience.

2nd DAM1021 build close to being complete, waiting for a 1-foot USB cable. This one is with the DAM1021-12 board, the one the uses .01% and .02% resistors which can be visually identified by the blue resistors in the R-2R ladder. The TPA Cronus/Hermes re-clocking and isolator is probably an overkill here given the DAM1021 already provides both functions on the board. The power supply for the DAM1021 is based on TPS7A3301 set for 9.3VDC rails. Based on my experience I would not skimp on the PS for the DAM1021. The extra power supply is needed for the Cronus/Hermes.