need opnion on my JLH15w PCB

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.

Attachments

  • jlh3.jpg
    jlh3.jpg
    47.8 KB · Views: 913
Calebay

I have not checked the track/device connections in detail but would offer the following observations.

I would prefer to see a separate return to the star earth point for the ground connections of R2, C2 and R6. That is to say, a 'clean' (signal) earth return and a 'dirty' (rail decoupling etc) earth return.

Space should be provided on the pcb for a heatsink on Q3. Even if a heatsink is not strictly necessary to keep the junction temperature of Q3 at a safe level, it will help to reduce the output dc offset variation during the warm-up period.

The feedback take-off point (R8) is not optimal in that it is taken from a point on the Q1c/ Q2e connection that is remote from the feed to the output. Although this is not as critical as for a Class-AB amp, performance can be improved by arranging the feedback take-off point as shown on the schematic (I don't draw the schematic this way just for the fun of it :).

I have had bad experiences of fuses and relay contacts in the feed to the speaker, but this is down to personal preference and your requirements may differ.

Geoff
 
Carsten

You have a pcb trace from Q1c to Q2e. At one point along this trace you have taken the feed to the output. At another point you have connected the feedback resistor. There will be a resistance between these two points which will cause the feedback signal voltage to difer from the output voltage and so distortion will be higher than it need be. See 'Distortion 7' (near the bottom of the page) in Doug Self's article for more detail.

http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

IMO, an output fuse should be avoided. If its rating is selected so that it blows before an output transistor fails then it will also more than likely also blow during normal use. If it is sized so that it doesn't blow during normal use then in all probability, under fault conditions, an output transistor will fail before the fuse blows. Also, I do not like the idea of having a modulting resistance which varies with temperature, time and signal voltage directly in the signal path.

With regard to relay contacts in the signal path, there may be better (hermetically sealed?) relays available now, but those I have experience in commercial products, many years ago, have all developed noticeably noisy contacts after a year or two and I am sure that the sonic degradation began long before the contacts began to crackle. If you do use a relay, remember that its contacts must be rated to handle any potential dc fault current, not just the ac currents present in normal operation.

Geoff
 
re relay in output

Hi Calebus,

your pcb looks nice but I had an idea about a mute -circuit which is configured in a different way.

I have no electronic image of the circuit yet but let me try here to explain in text.

The idea is
1: to have resistor of 100Ohms from the 'output' of the schematic to the speaker terminal.
2: use a relay which has 'alternating' contact. As I do not know the correct english term explanation is needed: with no voltage applied the relay makes contact between A and B; with voltage applied the relay makes contact between A and C. B and C are always separated.
3: connect the "A" point of the relay to the connection of the series resistor from step 1 and the output terminal.
4: connect B to ground and C to the 'hot' end of the resistor from step 1.

This results in a circuit when:
-no voltage is applied to the relay there is no signal across the speaker, and the amp sees a 100Ohm load.
-a voltage is applied to the relay all the signal is fed to the speaker.

Changing B and C in step 4 reverses the operation.

IMHO the major advantage is that on "crossover" from B to C the series resistor is actually in series with the speaker so it is biased before hitting the "full-power" position.

I think the value of the 100Ohm resistor is just up to your own discretion. A 10Ohm should work also. But care should be taken in the rating.
If only used for avoiding the "power on blob" (which my JLH-2003 has) power rating is not an issue. If used as mute (reverse B and C) the resistor should be rated as to (rail voltage squared) / resistor value.

So far from me.

Jos


btw: check my site for the Rev-E F and G of the JLH-ESL-2003 version pcb's.
my amp site
 
re possible hum

Hi Calebay,

my experience in two JLH projects has been that no humming has been found related to PCB layout.

I had however in both projects made the same arrangements:
- chassis-earth is not connect to signal-ground.

- the speaker-ground-terminal in both projects has been wired to the same nut and bolt which connects the PSU-ground to the AMP-PCB.

Succes with your JLH amp. It's wonderful
 
Calebay (Carsten): I checked the pattern against the schematic and all appears okay.
You may want to consider adding a spot for the feedback capacitor (C4) that has been removed for this version of the schematic, in case you want to add it back later. I noticed from the other thread you're having DC offset problems, although it sounds like you suspect oscillation.
Are you having this built or making boards yourself?
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.