Hey. Getting further along in my preamp design. Currently looking into the channel switching circuit. I will have a couple of logic ic's here and would like some tips.

I know that digital circuits might create some ground noise, anything i should think about when designing to minimize this? Or should i even use a small separate transformer even and optocouple the signals between circuits?

That means it should be the preamp that's the issue.

What i would do first is check for electrolytic capacitors with any kind of bulge. (although they can still be bad without it showing). Do the preamp caps look ok?

Alright. Read up a Little more on the amp.

Doing the tests i suggested would probably help alot to rule out the possible culprits.

1. The preamp
there is a level knob for your send jack. start with the level on the overdrive channel at 0, and the send level knob all the way down. connect the send jack to some kind of sound system. even the sound card line in should work as long as you're careful with the level buttons. it will sound like crap due to the frequency response, but you should be able to hear if there are any "dropouts".

Does it sound ok?

Yes
the preamp is probably not the issue.

2. so the preamp sounded fine?
Power amp
Get your DS-1 or a multi effect, Another preamp, a distortion pedal etc
connect your guitar to it, and into the return jack. start with the level/volume on the pedal low, and master volume low. adjust to non ear-bleed levels and listen.

Still doesnt sound right?

3. get/borrow an external speaker cab of 4 or 8 ohms, connect to your external output jack matching the cab impedance.

If this is where it works, the speaker(s) are probably broken.

4. Also, there seems to be quite some jack switching on this amp.
the external speaker jacks seems to disconnect the internal speaker.
the send/return jacks are also without switches, which probably mean that the jacks handle the switching there too.

Unused jacks (i've been told gold plated are less prone to this) tend to get bad Connections just sitting there. oxidize? cleaning or replacing them might do the trick?

Roly gave me this tip while i was working on a colleagues rocktron amp, and he sure was right. i Went nuts with the jacks, and it solved the problem.

WARNING:
I suppose that you COULD also disconnect the internal speakers and try them one at a time. but i really dont know what their impedance is and how they are connected, i strongly advice that you wait for one of the guys that actually know stuff here to comment on that Before you try. Speakers in combos are usually connected using spade lugs. which comes off quite easily
:WARNING

This is atleast what i would do. Once you know what part is causing the problem it gets easier troubleshooting (i work as a computer Tech/programmer, and this is more or less how i check a pc for errors. try one part at a time until i find the culprit).

I'm really no expert here. Experienced player, kinda new builder. so better wait for the "big guys" here to come with more qualified analysis. (mr Fahey, Roly, hint hint).

But just a thought.. are you sure that the speaker itself is ok? any chance to test the send out to Another amp? and Another preamp or hell even the DS-1 into the return? or disconnect the speaker, and connect to an external cab (Watch the impedance). As somewhat of an amateur, that's what i would try just to find which of those 3 is causing the issue, and move on from there.

Edit: By the way, wasnt that the amp that was modded by some Tech to get cooler? If that is the case, it would really help knowing exactly what was done. Maybe the mod has something to do with your problem? Did it behave like this Before the mod?

QuoteWhat i found was that chords on the Low strings sound fine, only the high strings are making that ping crackling noise. I tested it with 2 different guitars just to make sure its not my guitar.

Yeah, that's a good idea, the high E string tends to "oscillate" when it gets worn.

edit:
Never mind, i was listening through lousy speakers at work, i hear the problem properly now at home. you still need to change that string though

What are your settings on the sd-1? Normally to boost, you go with lowest possible gain, and max volume. if you run drive full on the pedal and then distort again through the preamp, squealing is more or less inevitable.

If you bypass the sd-1, does it still sound that way?

Thanx guys, really appreciate the tips and feedback here!

The more i Think about it, the more i lean towards going for a 2U case, but at the same time it would be awesome getting it to fit in a 1U. (going on europe tour november, and well, less space taken is Always nice, even if it's just 1U). On the other hand i really dont want it to fail either of course.

I'm still thinking about splitting up the 4 output transistors into 2 pairs on separate heatsinks, but also thinking about integrating the heatsinks in the enclosure. kinda like this:



On the other hand i guess there might just not be enough air around if i go with 1U, so i guess the tests are still on. just waiting for my Transformer to build the Power supply needed and a thermocouple-thermometer i ordered.

Again, thank you!

Quote{One little sidebar with the Thermo-Fan is that the top LED position can also be used to operate a cutoff or mute for those occasions where some schmuck hangs their coat over your rig.}

Hm, maybe i should cover it in barb wire Jokes aside, that's a good idea. will definately check it out.

joecool85: Yeah, i've read that post alot, i've also been Reading Teemu's book, and all heat sink related stuff on Rod Elliot's pages.

I Think i got the general idea for it all, but i tend to sidetrack ALOT in my Projects, as i just can't resist Learning how stuff works. But i'd like to Think that i could actually make something quite good when done.

Edit: Just ordered a bunch of different heat sinks on ebay in all kinds of SHAPEs and sizes, will try Everything from sandwiching the transistors between 2 sinks to putting them on individual and what not when i get my test rig setup for them. will try to get some semi-scientific numbers/graphs and check back.

Seems difficult finding a proper heatsink for this Project. Currently thinking about splitting up the transistor pairs, mounting them off-board on 2 different heat sinks on each side instead as that would give me some headroom finding heatsinks that would actually fit the 1U case. The amplifier board is tiny, so i got loads of empty space in there anyway. I Think i need to mash up some form of test-rig to actually measure the heat sink's efficiency. (yeah, probably overkill, but Learning and experimenting is fun!)

Thanx Roly, i'll have a look and see if i can find something similiar in europe/ebay. I do plan to add a fan, but i still need to be sure it doesnt BBQ if/when the fan gets bad, so going for a big enough sink stand-alone, and then adding the fan for extra cooling.

Hey guys, still researching and trying different solutions for my preamp (found in another thread, which have been on hold due to album recording and touring).

Meanwhile, i decided to buy a PCB from Rod Elliot for his 100W power amplifier, it was amazingly easy to build. now my plan is to put both the preamp and the power amp inside 2 separate 1U rack cases that i scavenged from old network switches at work.

Now the question i have is about the heat sink, i know rod has 0.5c/w written at the public page, but 1.0c/w at the locked page for buyers. i have bought this, as this was the biggest sink i could fit in the case, according to Rod's heatsink spreadsheet it's about 0.7c/w. But, i guess airflow wont be optimal due to the "layout" of the heatsink. You think i will get away with this one? or should i get an even bigger heatsink and put it in a 2U case? (i'm aware btw, that i will need to trim some of the sink off to actually make room for the toroid)

Thanx Roly, i have instructed him to test the amp with just a Cable in send/return. will see what the outcome is.

Hey. Been busy lately, but built a custom pedal for a friend, channel switching for his engl fireball 100 and a valvewizard smalltime delay.

He loves how it sounds, but when put in the fx loop (parallell, full wet) the volume gets lowered alot.

I know that line level loops can cause this, but accirding to the manual the send/returns are -20 to +/-0 with -10 nominal. So it shouldnt drop that much? Manual http://www.englamps.de/uploads/tx_ddfproducts/E635-OM.pdf.

The pedal does not distort, no input clipping and looks like unity gain more or less? But, as i read the schematic, the output is inverted, and being a parallell fx loop, maybe this is the issue? Or is it an impedance/cable length issue? But doesnt really hear tone degradation.

Schematic http://www.valvewizard.co.uk/smalltime.html.

Naturally i could just increase the opamp gain on the output. But i feel i want to understand the issue and use the least amount of band aid instead of proper design. If the issue is the inverted signal,i will add a volume pot and an another inverting opamp. Oh, and the buffer tubes should be fine, the amp is brand new.

Thanx for your tips guys, received my LDR's yesterday, so will start experimenting.

Thanx JM! i totally misinterpreted the data provided for them, had another look after reading your post and realized how it should be read instead.

Went ahead and ordered 80 pieces of 4 different values.

The log current source, i think will be easiest for this build to be generated in software for this project. but will take a look.