Hamptone JFP trouble

Hey there, i'm putting together a bastard version of the hamptone preamp from Tape-op and i've been having some problems! The main difference is that i'm using an altec 1588C as the input transfo, this provides 34 db of gain running at 18v and also puts out at 600ohms, and then a 15356A 600:600 bridging transfo for balanced output. Between the transfo's i have the two stages just like in the article. My problem is that the darlington in the second stage is overheating, it's not an MPSA14 but it is in a 220 package so it should definetly not be on the verge of melting like it is. I've changed the 10Mohm resistor at the source to 10K. I'm using 2n5089's as the bipolars and closely matched 2n5457's that bias slightly above 12v. I have 100K's in place of the 47Ks to the bipolars but the voltage drop across the 100ohm resistors to ground is still well above 4v(PRR said it should be at 4 or under) until i raise the values to around 220K which seems pretty high. This is kinda my first real experiment off the beaten track so i have no idea what could be going on. Thanks for reading all this!

I'd bet if you emailed Scott at scott (at) hamptone (dot) com he would definitely be able to lead you in the right direction.

It's worth a shot. :grin:

Ok a little of topic but; I remember when this thread started a while ago and everyone was talking about this thing being in Tape Op. Seemed like that issue took longer than usual to arrive and when it did I couldn't find the article. By that time I moved on to other things. So I guess my question is, What issue of Tape Op is this Hampton Pre in.

Sorry to get off track

Chris

The 1588C is a complete preamp in itself. Why cascade it into the input of another preamp, unless you're trying to build a fuzzbox? :wink:

hey thanks for the replies, the article was in the sep/oct 03 issue, i scanned in the two pages, you can check them out here:
http://members.cox.net/soma/hamptone.jpg (564kb)

I tried just setting up the 1588c and the 1536A as a preamp, but it's just not enough gain, prefect for a linestage, but i need more preamps as i don't use a mixer anymore. I was looking through the Altec 1592b schemos and there's a pretty hefty linestage in that mixer. I'm not an ee but as i understand it, a 1:10 xformer will raise the gain about the same amount as the 1588c so except for the impedance differences it should be similar?? Yesterday i took out the second JFP stage and tracked some guitar with a friend and it sounds pretty damn nice as it is, but i could really use just another 6-12db. I'm gonna try that || thing buttachunk, back to the bench I go!

Ahhhhh so that's where it has been all this time! Thanks Bro.

Chris

okay, just did this up as a di, and although it's working, the ztx is smokin hot, and the 1/2w 100 is startin to hate life(showin the signs of bbq), which is concerning me :grin:

will doing the || action on the ztx653 and mpsa14 keep this resistor in check, or do I have something else going on here?

regards

ju

I built some of these & in my opinion they definately need heatsinks.

I've built one of these (two stages as an instrument preamp/DI). Makes my bass sound freaking great! But, I traded emails with Scott about it because I was having some power problems. Turned out to be a combination of low current (needs at least ~400mA) and having a bad regulator. anyway...

Scott told me it's designed to run on 20V without heatsinks. But, the darlingtons on mine run really hot, too, even if I power it directly with an 18VDC wallwart (still sounds pretty good, tho). Apparently, he uses this circuit a lot, himself, and has quite a few out there that have never failed. But, I would have to agree... throw in the heatsinks.

~ Charlie

Alot of people in earlier discussions about this though it ran to hot, I am fairly sure that the kit ships with heat sinks. I also think the kit ships with some different transistors, judging from comments at the hamptone site. People have built the unit successfully with the published schematic, but I am not one of them. I had the individual gain stages working, but when I put them together I kept melting the ½ watt resistors (33ohm, i don?t remember) connecting the gain stages to the power supply, and eventually blew the voltage regulator).

There is discussion about it at http://www.hamptone.com/, but it is mostly by people who have purchased the kit.

Thanks for bringing this up, I hope to be working on it again in the next 2 weeks.

I´ve heard that some people are actually using transistors in parellel at the output to uvercome heating problems...

butta stated that up top, which is what prompted my :?: :grin: I added them in, and it did make them run a bit cooler, but still toasty =)

Gonna change ps ( and am orderin some heatsinks), as this one is only 300 ma

ju

Here's an excerpt from the hamptone site. I changed the resistor on the base of the ZTX653 to 150K, and it does run cooler. Should still heatsink it, but thought I'd pass this along:

Scott Hampton:

Ideally the ZTX603 and ZTX653 should have heatsinks. I have over a dozen of these 2xjfp boards in various prototype eq's. mixers, and stomp pedals, none of which have heatsinks, and not one has failed. If operating normally, the two transistors will feel warm, but not burning hot. If you think yours are running too hot, you probably have a ZTX653 with too high of a gain. You can remedy this easily by increasing the 68k resistor going to its base R3 and R10, to 100k or higher. At a point you will begin to clip one half of the waveform (you will hear distortion), so there is a point of diminishing return. If you can squeeze in small heatsinks on these parts (they are a TO-92 package size), its not a bad idea.

Click to expand...

I built 60 of these amp modules for my aplications so I have some experience with them.
The most important thing to do is to select the ZTX653 transistor. The beta should be not too high, something between 100 and 150 maybe 200.
Measure the voltage on the resistor from the emiter of ZTX653. The voltage should be around 4volts, this coresponds to a 40 mA curent that is Ie. If voltage is too high change the ZTX to a lower beta or increase the base resistor from 47k to a value that will give you that 4volts. I would choose to change the transistor to a lower beta than increasing the base resistor. Also I would use heatsinks because it is much safer.
All what I descibed is for 24 volt PSU.

chrissugar

Thanks so much for all the advice guys, i'm waiting for my order from mouser right now. I do have some really useful info that i printed from the orig Hamptone post that i'll try to get to and type up.

Seems kind of strange that a preamp meant to drive a somewhat higher impedance would require 400ma of current. I think I would look at how the output transistor is biased. All you need is perhaps 2 to 10 ma.

[quote author="BYacey"]Seems kind of strange that a preamp meant to drive a somewhat higher impedance would require 400ma of current. I think I would look at how the output transistor is biased. All you need is perhaps 2 to 10 ma.[/quote]

Well, I thought those things were meant to drive 600ohms impedance and maybe lower, like 60ohms...

If you are cascading two modules, like in the mic pre schem, it makes sense to do it with the first module.

Very little modern gear actually has a 600 ohm input impedance, even though this is what is advertised. I think the space heater effect could be re-designed to cut down on the kilowatt hours to the power company.

Yes, you are right. It makes sense.

But wouldn´t you need the full current if you wanna drive a 600/600 output transformer?

Not 400ma, unless something is very seriously wrong. A 600/600 transformer is only going to reflect the load impedance it "sees". If you are driving a recorder input as an example, that has a 10K input, that is approx. what the transformer primary is going to reflect , 10K. The only way this will ever have any demands put on it's performance is when driving a real 600 ohm load, and it won't take 400 ma of current to achieve that.

Even if you were to put a full 24 V rms across a 600 ohm load, it only works out to 40ma. This running full bore at clipping.