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 Post subject: good progress
PostPosted: Sat Aug 13, 2005 9:04 am 
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On the diamond buffer schematic I replaced the led ccs with a mirrored jfet css, parts of the pcb layout are changed and the minimum isolation distance is now 10mil. A small run of pcbs is ordered and expected to arrive Monday, August 22th

A perfboard prototype of the latest push-pull buffer design is built and running fine. pcb layout is almost done: single layer, discrete parts only (no smd), will be easy to populate. I'll order pcbs in a few days.


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 Post subject:
PostPosted: Fri Aug 19, 2005 5:48 pm 
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stienchen,

Thanks a lot for taking the lead on this. Any further progress, specially your impressions of the new variation of the buffers.


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PostPosted: Mon Aug 22, 2005 4:20 pm 
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everything on schedule :)

The diamond pcbs will arrive tomorrow, the push-pull pcbs are in production and with a little luck will arrive on Friday. I'll got all parts for populating the pcbs and I'm planning to post a first roundup next Monday.

I thought about part selection and want to replace 2n3904/2n3906 with 2n5088/2n5087 respectively BC550/BC560. For a bjt output stage at 30mA bias no problem, for a mosfet output stage at 60mA bias acceptable, at 80mA (too ?) close to their maximum rating. I allready tried BC550/BC560 at 80mA bias on a perfboard prototype with no problems. Since I'm a rather conservative guy I'm unsure whether that's a good idea in the long run and for the masses.

@amb
I wounder why you designed the current mirror for doubled current (on the push-pull design). I guess for keeping 2n5486 as jfet ccs. With lowZ cans I got low volume white noise, not annoying but audible. I'll give a 1:1 mirror and pn4392 / BC246A a try.

Assuming that everything works out fine we can start to probe interest and demand at the end of August in about two weeks. I'll be offline for two weeks in September due to vacancy, so we can schedule the finish of development for end of September.


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 Post subject:
PostPosted: Mon Aug 22, 2005 9:38 pm 
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steinchen wrote:
I thought about part selection and want to replace 2n3904/2n3906 with 2n5088/2n5087 respectively BC550/BC560. For a bjt output stage at 30mA bias no problem, for a mosfet output stage at 60mA bias acceptable, at 80mA (too ?) close to their maximum rating.

You lost me there... in the MOSFET buffer case the small signal BJTs don't flow the full current... The MOSFETs do.

Quote:
I allready tried BC550/BC560 at 80mA bias on a perfboard prototype with no problems. Since I'm a rather conservative guy I'm unsure whether that's a good idea in the long run and for the masses.

The max current rating of the 2N5087/5088 is only 50mA, and the BC550/560 is 100mA. You shouldn't operate close to the max rating continuously. Also, with half the supply voltage on each one of the output pair, the power (heat) dissipation issue comes into play. TO-92 devices can't dissipate much heat. For the 40mA case and assuming 28V supply, P = 40mA * 14V ~= 0.6W which is too much for TO-92 devices without heat sink (and since the TO-92 device doesn't have a metal surface, the thermal resistance is not too good).

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I wounder why you designed the current mirror for doubled current (on the push-pull design). I guess for keeping 2n5486 as jfet ccs. With lowZ cans I got low volume white noise, not annoying but audible. I'll give a 1:1 mirror and pn4392 / BC246A a try.

Yes, I was trying to keep the JFET current low. Increasing the current might make the noise worse. Maybe the particular 2N5484s you had were noisy? Try different ones to see if they make any difference. Of course you could also try 1:1 with PN4392/BC246 too.


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 Post subject:
PostPosted: Tue Aug 23, 2005 4:43 am 
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sorry for causing confusion, I shouldn't post after midnight :X

for the small signal BJTs I'm gonna try 2n5088/2n5087, BC550C/BC560C and 2n3904/2n3906

for the output stage the candidates are DB139/BD140, MJE243/MJE253, KSC2690a/KSA1220a and 2SC3421y/2SA1358y.

output stage mosfet: IRFZ24N/IRF9Z34N

Though expected to be inferior, I planned to try pn2222a/pn2907a with TN6715a/TN6727a out of curiosity. Since there are more important tests to do I shifted this experiment some weeks.

I missed to consider pads for a 220pF cap on the push-pull buffer for the upper mosfet, but the small cap can easily be soldered directly to the legs of the trannie.

To avoid confusion in the future I'll make new separate threads for the two topologies next week.


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 Post subject:
PostPosted: Tue Aug 23, 2005 3:42 pm 
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steinchen wrote:
I missed to consider pads for a 220pF cap on the push-pull buffer for the upper mosfet, but the small cap can easily be soldered directly to the legs of the trannie.

It's optional whether you use that cap or not. It serves to equalize the gate capacitance between the N and P MOSFETs, but its effect is all outside of the audio range so is a bit academic.


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 Post subject:
PostPosted: Sun Aug 28, 2005 10:51 am 
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update:

I received the two small pcb runs and assembled some sets, all working fine. Remaining todos are resistor value tweaking and transistor part selection.

I'm run out of small pots and will fetch some from a local store tomorrow to assemble some more sets with new settings.


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 Post subject:
PostPosted: Wed Aug 31, 2005 5:14 pm 
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pics

I've assembled some pcbs in various configurations, biggest difference (and improvement) was made by increasing the input resistor on the diamond buffer from 100 Ohms to 470 Ohms. I'm quite satisfied with the results so far, although I haven't tried the 2S trannies yet.

I've been running out of pots again, with $2.00 for each I'll soon run out of cash to burn, too. Due to the crammed layout and the smd parts changing resistor values on an already assembled board is all but fun. Nevertheless, I'm confident to finish the development phase soon.

The push-pull buffers with mosfets are a little problematic. You need at least 60mA bias to make them sound good, at 80mA the little heatsinks get pretty warm (or may I say hot). You'll definately need a ventilated case for them. Additionally, with mounted heatsinks, they reach up about 2/3 of the height of the tubes - very close or even above the tops of most cases used as far as I can see. Only few builders may be able (and willed) to install such huge buffers, I'm afraid. Of course the push-pull pcbs can be populated with bjt transistors with heatsinks omitted.

I'll provide the push-pull pcbs for the curious who want to give them a try, but won't spur on the topology. Nevertheless I'll finish the development (push-pull bjt + noise issue), of course.


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 Post subject:
PostPosted: Sun Sep 04, 2005 6:00 pm 
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beta tests will start soon and atm I'm working on a BOM and a howto (assembly, installation and bias adjustment)

BOM & HowTo xls (excel) [51kB]

afaik the breakaway strip pins are difficult to get in the states and I'm not sure whether the part I found at digikey is what we are looking for. the pins should have a diameter of 0.65mm (at least on one side) to give a tight fit in a dip-8 socket. as an alternative one could solder a dip-8 socket with short wires (e.g., cut excessive legs from resistors) under the pcb.

Comments, additions, part numbers from favourite vendors, report of typos, grammar correction (especially for my howto) are very appreciated


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 Post subject:
PostPosted: Mon Sep 05, 2005 12:36 pm 
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the file has gown to 263kB

http://stonerain.homepage.t-online.de/b ... pl_bom.xls

when it's done I'll split it into diamond and push-pull parts and create pdfs from each part


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 Post subject:
PostPosted: Mon Sep 05, 2005 5:52 pm 
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steinchen, in your BOM you list the mouser IRFZ34N with a ? mark. This device is not the same thing as IRF9Z34N. This is supposed to be a P-channel MOSFET but the IRFZ34N is N-channel. Unfortunately Mouser doesn't carry the IRF9Z34N and doesn't have a full selection of P-channel devices. That being the case, it's better to just drop Mouser from the list of MOSFET sources.


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 Post subject:
PostPosted: Wed Sep 07, 2005 2:54 am 
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removed mouser from list of mosfet sources :/

last night I investigated on the noise issue with the push-pull buffers

old config: 1:2 mirror, 2n3904/3906 transistors
with open 300ohms cans the noise was almost inaudible, with closed 75 ohms cans there was an audible noise, though the volume of that noise was so low that it was only audible when no music was playing

I changed the mirror to 1:1 and to BC550/BC560 trannies and the noise was gone on both configs, a BF245a (I ran out of 2n5486) was able to easily bias the mosfets, no need for pn4392.

Adjusting the mosfet bias with a 2k pot is pretty picky when the bias kicks in, the resistance needed for 60-80mA bias was about 400ohms most times, I think I gonna change the pot to 1k in the schematic.

Tonight I'll figure out the config for the push-pull bjt config and update the bom.


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