I have a World-design KT88 XL, The input level (sensitivity) with my Panasonic UB9000 media player is great, but the input from my Xiaomi mi box is really quiet and I need to turn it up nearly all the way to great a resonable volume when streaming video. Is it possible to increase the gain, just a little, without messing up the sound and stability?
A copy of the schematic is enclosed.
A copy of the schematic is enclosed.
If the required gain increase is not big, then maybe try reducing the global feedback by increasing R18, but be careful with stability. Maybe a switch to have a special "higher gain mode".
Another option is to replace R1 with a CCS or a gyrator, but that's a bit trickier.
Another option is to replace R1 with a CCS or a gyrator, but that's a bit trickier.
Increasing R18 will actually improve stability! This is even more true if C10 is decreased by the same ratio as as R18 is increased by. The tradeoff is higher distortion, but this may actually be preferred depending on how you like your sound.
On the other hand, don't mess with R1 especially increases. That is likely to be a problem with stability, and will have little effect with regard to increasing overall amp gain.
Rationale, in the first case, loop feedback is reduced and overall gain increases.
In the second case loop feedback will increase without overall gain change.
The more overall feedback, the more difficult it becomes to guarantee stability.
On the other hand, don't mess with R1 especially increases. That is likely to be a problem with stability, and will have little effect with regard to increasing overall amp gain.
Rationale, in the first case, loop feedback is reduced and overall gain increases.
In the second case loop feedback will increase without overall gain change.
The more overall feedback, the more difficult it becomes to guarantee stability.
Adding a switch for a "higher gain mode" would actually be rather easy on this amp. The input selector switch, actually has spare gangs that could be used to operate a relay(s) when a particular input is selected.
Considering that the voltage levels on this input are less, decreasing the loop feedback, rather than increasing the gain, is in effect, restoring it. So the level of distortion should be minimal. In addition, the input from the steaming box is hardly reference quality compared to my Panasonic UB9000, so a little more distortion is hardly to be noticed. Increasing the value of R18, as suggested by jcalvarez, with a switch, or relay would certainly not be difficult, decreasing C10 too, perhaps a little more so. My remaining question is by what sort of order can R18 be increased, for example, could I get 20% extra gain? Changing the value from 3.9K to 4.9K (adding 1K in series) what do you think?
There are a couple of ways to get the value for a particular gain increase:
Easy one: Add a potentiometer in series with R18, and find the value that gives you the desired amount of gain
A bit harder: Find the open loop and closed loop gain values, then you can find out the NFB level, and you can then calculate the required resistor value.
I have an amp that has a similar design, follows the EICO HF-89. R18 in my case is 5.1K, I get full power (28W avg) from 0.7V peak input voltage.
It's an attempt to balance the phase splitter. Because the 5687 has quite low mu and the tail resistor R6 is only 10k, with equal valued load resistors the left half would have greater gain than the right half. The R7/R8 combination gives 30k compared to 39k for R9, the maths will likely show that gives nominally equal gain. If R6 is replaced with a CCS the load resistors could be equal.
As tikiroo has said, it's to balance the top and bottom halves of the output. Possibly, in this configuration because the correct value is an odd one, or maybe to allow easy custom adjustment. Constant current sources in this design are avoided to try to keep the design as traditional as possible (but would obviously be more precise). And, although the power supplies are linea with solid state diodes (to reduce hum), the rest of it is pretty traditional.
Took a look inside my baby yesterday and noticed that the capacitors on the output stages don't seem to be faring very well. Certainly some heat damage, it’s be in service for just over a year. I am not very clear on the construction of these caps, but I am inclined to think that they are going to need changing and may need some protection from the heat. Opinions, please?
😆 No, I didn't burn them with the iron, they were in perfect condition when I put the lid on. Picture is just before the lid went on.