I did not win that bid but picked it up from the site posted by Mike. I hope to roll in some op-amps in lieu of the NE5532. So do you all think that the 4562 a pair of 797 or the OPA111 I got from Mike would work here?
Miguel, Don't know whether it is relevant but the metal can versions of the OPA111 are the best op-amp (to my ears) in my Beresford Caimanised 7520 DAC and my battery powered portable C-Moy HP amp. In both applications I hear distinct improvement from the metal can variety over the square 'plastic' version of the OPA111. FWIW there has been a guy on eBay, HK based IIRC, selling them in pairs mounted on a 2 into 1 adapter but I can't see them ATM. Dave.
Miguel, And have you compared the metal canned OPA111s against any others and if so, what did you think? I'd be interested in your opinion, either way. Just in case you are not enthralled with them I might be interested in taking them off your hands if they came with a 2 into 1 adapter. Don't want to trouble Mike ATM . TIA, Dave.
Miguel, I bought a Mini-T amp from a guy in HK/China some time ago - 2 days ago he posted me the third one as the previous two got stuck/lost in their postal system. Apparently (he tells me) their authorities have put restrictions on shipping LiPo and LiFePo batteries so all containers are being emptied and searched for these, causing huge delays. This might explain your delay . Dave. EDIT: - FWIW this is the seller involved - I've bought a few items from him (Peter Cheung) and never had less than exemplory service and good value products: -
The amp arrived while we were away on a 10 day long vacation. I picked it up last night and upon first turn on it was horrible! Plus …
1. It gets quite warm bordering on hot, I would not carry this in a pocket since I would be afraid of getting burnt. 2. The volume pot is very noisy when raising or lowering the volume. Once you set it up it is fine. 3. The battery makes a physical noise so I took the amp apart and used some putty to quite it up. 4. I used the same putty to dampen the tube. Contrary to the Millet TTVJ portable tube amp the tube does not make noises when the amp is bumped. 5. The newer amps like the one I got has a socket to allow rolling of op-amps. 6. It takes about 3 hours to charge and the amp will run about 3 hours on a charge.
The unit has run for about 4 hours and the sound is a lot better. The top end is not very extended when compared with an iBasso D4 and the bass is a bit loose. At about $62 it is ok as a conversation piece and unless it improves a lot (doubt it) it is a paper weight. Perhaps an LM4562 will improve its sound.
I have one of these units and it came with 22 ohms. It seems if this value is reduced (11 ohms) it affects the tube voltage and somehow improves sound. The one shown in your picture has 2 resistors in parallel - perhaps yours doesn't?
I changed the pot in mine and I'm actually quite happy with it.
Dicky, below is a link to the pics I took of the board hope it helps. The amp sounds ok when you just relax and listen to it. Once it is compared with other amps you hear it's shortcomings. It is at about 5 hours of use.
Post by joethearachnid on Sept 10, 2012 23:05:53 GMT
I would hazard a guess at the biggest resistor at the far volpot side of the board. I'm too lazy to actually check the stripes. Dicky, what does that resistor actually do? It'll be interesting if it improves the sound enough to make the amp worthwhile.
Miguel don't worry about it. The resistor in your amp is 10 Ohm already. In the original (the picture on ebay) they used two 22 Ohm in parallel giving 11 Ohm.
The resistor is in series with the heater and some other resistors to supply the heater with 6V from a higher (12V or 9V ?) battery pack. Given the values I fiugure the battery pack would be 9V unless some other resistors also get hot, in which case 12V could be used.
The amplifier is a simple C'Moy (6 Ohm output resistance) with a tubefollower in front of it. I take it it uses the 6BF7 (which is a 300mA tube at 6V) meaning it will draw 3W ! just for the heater where the amplifier part itself simply draws a few mW (I am guessing 9V battery).
Heat can be improved by using a switchmode regulator for the heater. It will drop to 2W increasing battery life with 50% and lower heat output. Also the opamp has NO local decoupling caps at all. Certainly when opamp swapping and using fast opamps you are asking for oscillations.
A LOT can be improved in the design but I guess we all have gotten used to that idea by now with cheap Chinese designs.