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Topics - joecool85

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Schematics and Layouts / Fender Frontman 25 DSP FM25DSP schematic
« on: July 12, 2017, 01:17:43 PM »
Fender doesn't have this file on their site anymore, and I just had to track it down (thanks  So I figured I'd share it here.

Unfortunately, Teemuk's thread was deleted.  I have made this post to at least have the link available to everyone.  I apologize for any inconvenience.

Quote from: Teemuk
My book about solid-state guitar amplifiers is finally ready and can be downloaded in portable document format (pdf) from:

Big thanks to joecool85 for hosting the file.

People with slow Internet connection, be warned: The file is about 31 Megabytes so it will take a while to download.

The K-20X doesn't have a line in.  What this means is that to play to a backing track I need to have a CD player or radio etc playing over a speaker while I play my guitar over the amp's speaker.  This works but is clunky and can also get loud.  Rather than purchase a Vox Amplug or similar ($40 or so and have great reviews, I still might do it at some point), I decided to go the cheap and DIY solution and raid my parts drawers.  What I found:

1 x small wooden box
1 x 1/8" stereo jack
1 x 6' stereo 1/8" cable cut in half
4 x 200 ohm fire-resistant 1w resistors (1/8w would have worked and they don't need to be fire-resistant, but it's what I had)

With this I made a passive mixer designed for headphone level signals (the same design can work with line level signals but use 10k resistors). Now I plug this into my amp (with stereo 1/8" -> 1/4" adapter), and cell phone, then plug my headphones into the passive mixer box and BAM! I'm rocking to tunes and not bothering anyone in the house. If only somehow my singing could be dampened...

I don't have a schematic, but the gist is this: For each channel coming from each device you put a resistor in line with the positive.

So Device One's left positive goes into a 200 ohm resistor.  Device Two (amp) has it's left positive go into another 200 ohm resistor.  Both of these resistors terminate at the left positive of the headphone out jack.  Do the same with the two right channels, tie the grounds all together and you're done!  Easy peasy and works a treat.

Amplifier Discussion / My K-20X
« on: February 27, 2014, 12:17:45 PM »
I've been playing guitar for 12 years now and the whole time I've owned this amp - Dean Markley K-20X.  It's been through a lot.  Over the years I have added a line out (just like the K-20, not sure why they got rid of it on the K-20X), added a speaker out jack and removed the rear wooden "bar" on the back side of the amp.  I now have replaced the permanently attached power cord with an IEC jack w/fuse.  Not only does this allow for quick cord replacement, but also easier storage, the use of a 90 degree cord so you can put the amp closer to other devices, and it puts the fuse in an easily accessible area (back panel).  The old fuse location being on the PCB was not only inconvient, but arguably unsafe (putting mains current and voltage to the PCB).  The mains wiring now bypasses the PCB completely making the amp safer and easier to service.

As a side effect, I needed to relocate the power transformer to have room for the back side of the IEC jack.  It is now directly under where it use to be, same mounting holes and all.  This puts the transformer outside of the metal enclosure.  This potentially removes some noise (though so far I haven’t noticed a difference).

Now the pics!

Amplifier Discussion / Holmes Tech T-25, DC at speaker
« on: February 24, 2014, 09:16:52 PM »
I'm working on this old T-25 amp and am getting 17.3v DC at the speaker while connected to a lightbulb limiter. The Poweramp utilizes a tda2030a. I think it's blown, what do you guys think?

Of note, the mains wiring on this had been an unsafe mess. I've fixed that, now just need to make the now safe amp work...

Amplifier Discussion / Whoops...messed up my line out voltage divider
« on: February 17, 2014, 01:41:29 PM »
Years ago I added a line out to my k-20x by using the 10k/1k voltage divider the k-20 used. Except evidently I wasn't paying full attention when I did it. I put the 10k inline from the positive speaker lead to tip on line out, then a 1k from the negative lead to line out sleeve. I should have put the 1k from tip to ground to complete the voltage divider. Luckily I only ever drove a lm 3886 with it. I'm going to fix this soon as I'll be in there anyway doing an IEC jack mod.

Amplifier Discussion / Adding a balanced line out?
« on: March 13, 2013, 01:59:24 PM »
My brother has a Hartke Bass Head (I believe the 2500).  He wants to add a balanced line out.  He had an issue recently where the sound guy wouldn't hook him into the PA because that was all he would work with, wouldn't take the preamp out (effects send).

Anyway, is there a way to add a balanced line out?

Amplifier Discussion / Snap disc switches - thermal management
« on: December 19, 2012, 12:02:17 PM »
I just installed a snap disc to trigger a shutdown of our servers at work.  These things work all the way below freezing and up to 300+F.  Tons of different mounting configs and temp ranges.  They go for about $10 each and are made in the USA.  I'm thinking it'd be real easy to install one on a heatsink and have it shut the amp down or turn on a fan.  No added noise in the circuitry and keeps it simple.

Check 'em out:

Amplifier Discussion / Tech 21 Gear still Made in the USA!
« on: July 25, 2012, 08:59:05 AM »
Wow, how did I miss this?  It appears that Tech 21 gear is still all designed and manufactured in the USA.  I might need to take a look at the Trademark 30 and 60 amps...

Amplifier Discussion / Audio simulator software
« on: July 13, 2012, 08:09:24 AM »
I've seen that you guys have talked about simulating circuits, especially preamp pieces, to check for frequency cuts/boost etc.  What do you guys use to simulate these circuits?

Amplifier Discussion / Diago LS01 - 5w 9v amp?
« on: June 19, 2012, 11:45:55 AM »

Anyone have any idea what it uses for a chip?  Also, I doubt the 5w rating considering it asks for 9vDC @ 600ma which is only 5.4w...must be a VERY efficient design, no?

Schematics and Layouts / 12v DC Fan Controller (LM317)
« on: May 29, 2012, 12:36:04 PM »
Alright, I've come up with a fan controller that I'm happy with.  It can run up to 1amp of current for a 12v fan so long as it is heatsinked.  With the fans I've played with, 4.5v is when they turn on and start blowing any sort of noticeable amount and 12v is full tilt.

Circuit Approximations:

When R3 = 0K
4.5v happens at 40C/104F
12v happens at 75C/167F

When R3 = 5K
4.5 happens at 28C/82F
12v happens at 63C/146F

When R3 = 10K
4.5 happens at 18C/65F
12v happens at 55C/131F

Notice how as the resistance increases, the turn on/full tilt temperatures decrease.  To make this more natural in how it works, I would wire the pot backwards so that all the way clockwise would be the hottest temps and all the way counterclockwise it would turn on the soonest and run the coolest.

Schematics and Layouts / Joe's simple fan control - untested
« on: May 17, 2012, 04:07:52 PM »
**This circuit is untested, unverified and not recommended for anything other than testing purposes!**

This is what I came up with for a simple temperature driven fan controller.  It is a fan controller I worked on, based on the Temperature Controlled Fan I found online.

This is good for a maximum of 1.5amps, but really you should keep it below 500ma to keep it reliable.

In this version, R1 is a 10K NTC thermistor (shown in schematic at 20C, which is 12.5k).  Adjust R2 for the turn on temperature and R4 for the full speed temperature.  Fan is a 12v PC fan that draws 0.11 amps.

Note that if you change R4 (full speed temp) you will need to re-adjust R2 (turn on temp).  Due to this behavior, I suggest using trim pots for R2 (5k) and R4 (500R) and adjusting R4 (full speed temp) first and then R2 (turn on temp).

Circuit behavior also changes upon load of fan, IE 0.11 amp fan will have different temp ranges than 0.25 amp fan.

*Note: I have not actually built this circuit, only played with it in LTSpice.

**Note #2: I just noticed in some more simulations that the values change if the fan isn't exactly 0.11 amp, I don't know in practice how much it will matter, but it is something to consider.  For a controller capable of high current (up to 1 amp) that doesn't care what the amperage of the fan is, see my new 12v DC Fan Controller (LM317)

Amplifier Discussion / Dean Markley back in the Amp Biz
« on: January 31, 2012, 04:19:54 PM »
Apparently I missed the memo, but Dean Markley is back at it making amplifiers again.  Very cool!  They bought out UltraSound in 2007 and then in January 2011 they started making their own amps again including some tube re-issues as well as a nice lineup of solid state gear.

Here's a link to their page:

The DM60RC looks like a nice amp with a 12" speaker, 2 channels, reverb and chorus all footswitchable.  It uses digital reverb which is too bad though.  The DM15R and DM30RC both use spring reverb units which is cool.  The DM15R has no footswitch options, the DM30RC can only switch channels (not reverb and chorus like the 60). 

They are more pricey than Fender Frontmans and similar, but so far reviews have been that they sound WAY better and seem to be built a lot stronger.  I gotta find one and try it out, they look nice.

MSRP pricing can be found here:

Amplifier Discussion / Fender Champion 30 - Low volume, lots of hum
« on: November 19, 2011, 06:45:58 PM »
I'm working on an amp that has me a bit stumped.  It is a Fender Champion 30 and it has hardly any volume passing through from the guitar to the speaker regardless of which channel it is on.  There is a good deal of hum/static as well.  The volume does work, but even zeroed out there is quite a bit of hum coming through the speaker. 

What's more weird is that when you shut the amp off, there is a split second where the amps sounds pretty loud and relatively clean - I'd dare say pretty much normal.  I've only looked at it for a few minutes and did some quick voltage checks and things appeared to be in order.

Any ideas?

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