The amp just arrived. Without much delay. I had tested it out and the amp was very hot during operation. I had stopped it immediately after about 15 mins of listening.
Opened up and done some measurements and had noticed the voltage for the filament was 11.43v. I was shocked! Really need to install the TSR soon....
Also noticed that 2 of the resistors (directly under the tube) were soldered only with one lead on the board. The other leads of each resistor show signed of being cut. Then, in the same positions, similar valued resistors but bigger were soldered on the under side... put them underside is to prevent heat damage by the tube I think...
Furthermore, not sure the reason for adding another one smaller resistor on the underside for? See photo below...kind of strange...
Looking at the resistor for the filament, it seem to be 1 ohm? Why they only use 1 ohm resistor? For most case at least they use a 10 ohms. Not sure how long the tube will last in such voltage. The tube used was Sylvania 6111. Thanks.
Hi, I would like to use the LM4562 and AD797 on this amp. However the NE5532 op-amp can take up to 22 volts versus 17v and 15v respectively for the LM4562 and AD797. Does anyone know what voltage is being supplied to the op-amp in this amp? Thanks.
Ok, I took a chance and did a quick and dirty op-amp swap. I used:
- Two AD797 in a dual to single board. - AD8620 - LM4562 - Stock NE5532P
I found the sound from both the AD797 and LM4562 kind of dead and with less liveness to the sound. The AD8620 was close to the NE5532P with perhaps an slight sound difference so I left it and will listen to it over the next couple of weeks.
BTW I tried the "water towers" for a few seconds and liked the sound, unfortunately they would not clear inside the case so I took them out and did not pursue them since it would not be possible to used them as they are.
Hello folks. I'm new in this forum, but i've also did some mods with the small Little Bear tube head Phone amp. First of all, i've bought from eBay a little DC/DC step down convertor (http://www.ebay.nl/itm/400362544457?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649#ht_2611wt_1393. This one is much easier to handle because of it's dimensions. 2nd, i missed something. somewhere in this forum, the tube is changed from a 6111WA into a 6BF7. Is this possible without changing something else?? Please give me a reply if you know the reason for this tube rolling. Thx.
International Standard IEC61938 requires that headphones should be driven by a 120 ohm source,regardless of headphone impedance.This applies to many headphones, including the AKG K701, Audio Technica W1000 etc. You can't drive these types of headphones correctly from a portable amplifier. Mike suggested the Panda as it has supply rails of + and -20V and is capable of driving such headphones from a 120 ohm source impedance. This would be a 120 ohm resistor at the output of the amplifier. Unless this is done they will not be able to be driven to satisfactory levels, and will have a fatiguing and exaggerated HF response. Alex