AMB Laboratories DIY Audio Forums

by **Hasren** » November 4th, 2019, 1:58 am

I would have loved to start with the background of my built, pictures and lessons learnt, but I have hit a brick wall requiring urgent assistance from the astute builders that have helped me a great deal through reading the forum. Trust me I have been through a lot but managed to find solutions from the forum. So I am at a stage where I feel I have to ask before taking certain decisions. I am at the initial set up stage for the second time (after having the perfect one and blew it by a stupid mistake - swapping the unregulated polarity and blew the whole thing up). Had to replace a lot of components and the circuit is stable i.e. no more smokes or excessive heat.

1. Unregulated Voltage = +45.1V and -45.2V
2. Regulated Voltage = +37.3 and -37.1V
3. Input stage bias setup: easy to set to 4V
4. VAS stage balance setup: can attain 130mV balance for a short while but very unstable fluctuates, in fact, all the other steps start to behave this way and couldn't even move to Null the output DC offset.

I have tried to swop Q1, Q3 put them back and tried Q2, Q4 I have cannibalized the other channel (by the way the other channel is now dead from cannibalism :twisted:

) hoping that the JFETs are still intact. Still no luck. However, when I analyze the behaviour of the various combinations I am inclined to conclude that the issue is with the JFETs (some behave better in that the fluctuations are much more stable than others but still there nonetheless) . Could this be correct? If so I intend ordering new freshly matched Quads (from AMB). By the way, I am building Monoblocks.

by **amb** » November 4th, 2019, 12:13 pm

While you're making the bias adjustments make sure not to touch any of the devices with your hand or other objects, or thermal effects will cause the measurements to drift. If you want to test the JFETs, remove them from the board and measure their Idss and see if they are sane (procedure can be found at the β22 website under "Device matching"). You may also want to check their transconductance (or "gain"). Search the web for "measure JFET gm" for info).

by **Hasren** » November 4th, 2019, 12:43 pm

Thanks, Ti, I will try that but I feel I have just messed up the combinations of the quads and I am afraid the board might get damaged further hence I was thinking just starting afresh with newly matched JFETS. I will give it a try one more time.

by **Hasren** » November 9th, 2019, 11:16 am

Sorted

...guess what? loose connection Q40 :lol:

by **Hasren** » November 9th, 2019, 11:25 am

Another happy B24 builder Ti...The fact that I could complete the project through reading from the forum is proof that this project is matured.

by **Hasren** » November 9th, 2019, 11:32 am

The inside Construction...Unregulated Trafo = 300VA Novutem Telema, Regulated 15VA Swift (Local).

by **Hasren** » November 9th, 2019, 11:48 am

Getting ready for the second Channel...

by **amb** » November 9th, 2019, 12:16 pm

Excellent!

by **Hasren** » November 16th, 2019, 2:18 am

Quick question: As you can see, I have decided to take out the LEDs (located on the front of the chassis) but the problem I have is that they are very bright. I know I may choose to use the less bright option but I still love the glow and the feel of the brighter option (it looks beautiful). So my question is, can I increase the values of RLEDs from 10k to say 100k? Will this have a negative impact on the operation of the amp? Lol, I know this is a nice problem to have because the amp is playing beautifully :lol: