My JLH 1996 EVO....output transistors questions.... - diyAudio
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Old 24th February 2012, 09:33 PM   #1
AC439 is offline AC439  United States
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Smile My JLH 1996 EVO....output transistors questions....

(Please don't kill me saying there is another JLH thread. I have been reading that thread day after day and my wife is about to divorce me and let me marry JLH! I learned a lot from that thread but it is out of control and has 227 pages !)

After my all MOSFET amp disaster, I focus on the JLH. I have just built the 1996 EVO based on Rudy's schematic. Simple and no CCS, keeping the boost strap etc... My goal is also to use as much parts as I can find from my parts bin. Only thing I bought are fake Toshiba output transistors because my 2n3055s are all different brands and not getting a match for all the fours I needed. I want to make sure it works before I put better transistors.

I put the prototype together one channel at a time using a pair of original motorola 3055. The trans have great gain and good sound. I ran the prototype off around +-13.5V DC (from a small 24v CT transformer). Then I replaced with the fake Toshiba. They have very little gain (around 10 hfe) so I have to adjust the current up until above 700mA or so before it started to sound good.

Today I moved the prototype into the enclosure with a bigger 16v CT tranny so I'm getting approx +-23v dual rails. I have the Iq set at around 1.3A now. I have to use active cooling to keep the output transistors temp in control. 0mV DC offset without drifting.

Just as I think I'm done and am listening to the amp, I just feel like the amp does not sound as warm as when I ran it with +-13V with a little lower Iq. (with same speakers and same source)

I know at this time you're probably about to say change out the fake toshiba (which is what I'm planning to do) to 15003. I still have to order some more thermal grease while I'm ordering some 15003.

But my question is - "Is current always the higher the better? Is supply voltage always the higher the better?"

Also, if I use 15003 in place of 3055, will the heat reduced due to the bigger power dissipation from a bigger size semiconductor chip?

This question is about paralleling output transistors: Say, if I have a single transistor and set its Iq, say at 1A to put it in pure class A. And say, the same transistor will be in class AB when its Iq is 0.5 A. Then I parallel two of them, keeping total current to 1A and each transistor carries 0.5A. Will the final product in class A or class AB ? The reason I'm asking this is I have read people paralleling transistors to reduce heat generation and increase output power. I'm wondering if letting two transistors sharing the current will make them running in a less than linear region compare to just use one transistor and set it at a higher current.

Thanks.
- AC
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Old 24th February 2012, 09:51 PM   #2
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I built a 1996 JLH and it's awesome!
I used 15003s and +/-16.5V rails. Iq is between 1-1.5A (depending on how I've been tweaking it at the time).
I feel that any less than 1A and it doesn't sound as good.

If you're 'underbiasing' it, it doesn't force it into class AB. I think it sort of clips one cycle (and the DC offset adjustment would be tweaked to centre it around 0V). I'm sure someone can define more accurately what happens, but you definitely don't get crossover distortion like in class B/AB.

I think you've answered your own question, moving to MJ15003 should solve your issues. Mine sound fine at 1A (I haven't paralleled mine).

If the 3055 is a plastic package, and you're moving to a metal can MJ15003, then yes, the heat transfer should be better.
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Old 24th February 2012, 11:04 PM   #3
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69JLH is class A only, will not do AB at any bias setting.
Current steering with BJTs defines class A with a hump
in the middle that is totally opposite the dip required for
AB operation. When driven to extreme it simply clips.
At no time or setting will it go AB. NEVER!

The hack to define an AB style output class requires a
non-linear pre-shaping of total available drive current
before JLH steering. Shunting away excess bias, beyond
that which defines your chosen output class, is probably
the easiest way. The choice of bootstrap or CCS to feed
such a bias shaping shunt is a matter of personal taste.
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Old 25th February 2012, 12:46 AM   #4
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by AC439 View Post
But my question is - "Is current always the higher the better? Is supply voltage always the higher the better?"
The main issue with higher current and/or higher voltage is power dissipation. Otherwise, my feeling is that higher voltage is usually better. I don't have enough practical experience to comment too much, but I would expect higher voltage to lead to lower distortion.

The thing about 2n3055 in terms of current is that you want to avoid the current being so high as to enter the region where these devices have excessive beta droop. All BJTs suffer from this to different extents. So parallel devices like you are planning to use will allow each device to operate at lower idle currents where the gain is still good.

I plan to stick with 2n3055s for mine and operate each device around 1/3A with 4 devices in parallel for a total of 4/3A.
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Old 25th February 2012, 03:56 PM   #5
AC439 is offline AC439  United States
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Thanks guys for comments. I will definitely go put 15003 later and set around 1A as Phil suggested. And thanks Bigun for explaining the paralleling transistors. I didn't pay much attention to beta droop but now I understand why. My fake Toshiba has very low hfe (around 10 or 11, I tested them on breadboard). So when I raised Iq to 1.3A, it further dropped the beta and the sound became compressed. Ken, appreciate your input but I don't think I get it.

So, I lowered the Iq to 1A today. The sound is fantastic ! Even with fake Toshiba. Already better than my MF A1s. I think with the toshiba, anything higher than 1A will make the hfe too low to sound right. Anything below 700mA also make the amp sound muff and dry (even my wife can tell). I have been listening to it now for 3+ hours and not feeling tired. Another good side effect is the heat dissipation has reduced with less Iq. But still needs active cooling. I have some silent computer fan which I can hardly hear it running and it does the job well. The fan in the pic is an 120mm but I do have silent 80mm fan to install. As you can see from the pics, it is still not completely finished. The enclosure is too big I think. You can also see I use 2n4232 as the phase splitter. It doesn't have a high hfe (around 50) but works fine. The input PNP is a no name type that a relative passed onto me 35+ years ago (hfe around 170). The boost trap cap is phillips (blue) and the input cap is a 0.68uF (westcap, yellow). Bass reproduction is not a problem with this yellow cap. You can see I mounted the 4 toshiba to a L bracket then bolted it at the back with heatsink. Then I put a fan over the heatsink. It works well. Just not sure if heat will be a problem when I have the top panel install later. Power supply is a simple 36VCT6A transformer with bridge rectifier on two 6,800uF caps. There are just a little hum when I am very close to the speaker. C-R-C filter will be installed later.

Sound is great. Very very warm, especially the female vocal. Bass is very strong. Clarity is surperb in the entire frequency range. I'm only using my ipod as source with MP3s I ripped or internet streaming. It has already surpassed my expectation. I'm not even sure what will happen if I change the 3055s to 15003s (maybe making my hairs at my back standing straight ??).

- AC
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Old 28th March 2012, 10:08 PM   #6
AC439 is offline AC439  United States
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Smile Few more pics here......

I have done a little more on the amp mostly finishing up and here's a few more pics.

I mounted a front power sw with built in light (left) and a volume pot (right). Then I permanent mount a 80mm silent computer fan (got from newegg) powered by a cheap 12v wallwart mounted inside the enclosure. It keeps the output transistor at a good temp and will not overheat.

I found myself enjoying the amp everyday even just with the low hFE fake Toshiba. (Haven't receive my 15003 yet). My other remaining minor issues are trying to add a pair of filtering caps to form a CRC filter to get rid of the tiny hum from power rails and a slight vibration top panel due to the location and close proximity of the power transformer (links below).

I thought it was a myth...
Chassis vibration ! How to solve ?

Rarely do I fall asleep while listening to music. But this JLH amp sounds so warm and soothing and it repeatly put me to dreams everytime I have it turn on !

-AC
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Old 28th March 2012, 11:20 PM   #7
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AC439: What is the background on the 'fake Toshibas,' and why are you taking your chances with them?
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Old 29th March 2012, 01:34 AM   #8
Bigun is offline Bigun  Canada
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oh boy, are you hooked now ! another person lost to DIY !!

I have just finalized my take on the JLH design (I think the 1996 version with CCS) by verifying how it works with Spice and will begin building soon.....
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Old 29th March 2012, 01:46 AM   #9
AC439 is offline AC439  United States
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Initially I have two pairs of antique Motorola 2n3055 but I found one of them has leak between C-E. I went ahead and use the good pair to finish two channels (one at a time). I have other 2n3055s but they are of different brands. I want to finish the project cheap so I got the Toshiba from the "@bay" (less than $1 a piece) because I want all four of them same brand. They tested out with low hFE, and I plan on changing them once everything else is completed. I had to tweak the Iq, keeping it at around 1A. To my surprise, they sound very good (but I think the antique Motorola sounds a little better). So, I'm a bit slow to change them to 15003.

In fact, the amp sounds very musical and much better than my Musical Fidelity A1 with preamp mods. It is the best sounding amp I have ever built or own (haven't build any tube amps yet, but have three old tube radios and love them). In this regards, I really have no complaints with the fake Toshiba.

Last edited by AC439; 29th March 2012 at 01:48 AM.
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Old 29th March 2012, 01:50 AM   #10
AC439 is offline AC439  United States
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Quote:
Originally Posted by Bigun View Post
oh boy, are you hooked now ! another person lost to DIY !!

I have just finalized my take on the JLH design (I think the 1996 version with CCS) by verifying how it works with Spice and will begin building soon.....
Yep, Definitely hooked !
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