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1970s Design Indulgence

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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 15 Jan 2020 at 8:34pm
I was amazed, after a bedding-in period, to find the same sound from the trans-diodes as I'd got from the transistor bias spreader, and not a good sound at that - measurably fine but audibly sibilant.

The difference between a base-collector connected transistor-diode and a normal diode is the maximum reverse voltage, which is about 5V for a transistor, and about 100V for a 1N4148 diode.

The only thing I can imagine it being is that the reverse biased transistor zener behaviour generates noise - often taken advantage of in synthesisers to produce a particular quality of noise - and somehow, even though forward biased, there must be some charge at high frequencies due to the dynamics of the music. This might sound like me clutching at straws, but at this point I'm lost for any other explanation, so if you know better, please say!

A 1N4148 diode glued to each power transistor works fine, and doesn't seem to have this problem, and it might be just coincidence, but it doesn't act as a zener, and even if it were to, it would be back biased at 100 volts.

The trouble is the glue: I would much prefer to have the thermal detection device mechanically fastened, but a 1N4148 is such a finicky thing to bolt down without risking it breaking or shorting to a lead.

There are other ways of using a transistor as a diode, so I'm off to look at them.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote peterb Quote  Post ReplyReply Direct Link To This Post Posted: 15 Jan 2020 at 10:48pm
Would an SMD version of the 1N4148 stuck with a thermal conductive glue make it a bit easier?

Edited by peterb - 15 Jan 2020 at 10:49pm
Peter
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 17 Jan 2020 at 7:16pm
There are imagined, measured, and observed factors in amplifier design. I try to avoid the imagined, try not to rely on the measured too-much, but trust in observation.

People say if it sounds good it's right. I once made a simple phasing device which worked its magic whilst turning the control knob. It sounded good, but as a method of reproducing music accurately, it wasn't good.

I have listened to unstable amplifiers in audiophile systems and heard the occasional sound appear as if it is somewhere beyond the listening room, but other sounds remain within the walls. It can be a wow moment, but it isn't right.

I have heard Hendrix whilst playing walk round the back of the room, and that's either a clever or incidental studio panning trick, which in the "right" conditions works, but more often than not, doesn't.

Tuning by ear in such ways might "improve" some music, but often at the expense of other music. I think if you observe sufficient performances of music you learn the sound of instruments, and how they sound in venues, and how sound reinforcement, and its tricks, makes it really sound.

Then when you listen to similarly recorded music reproduced by the electronics you're working on, you have a good enough idea whether it's doing its job correctly or not.

Live music can be a full orchestra in a concert hall, or a nephew playing his electric guitar on his practice amp in your back room. It can be the organist and drummer at a social club, or a rock concert at an arena - it's all live - and it all has its artistic persuasion.

The organist and drummer demonstrate accidental musical interactions, where the low organ notes rattle the snare drum-skins, but in a studio recording where instruments are separated this obviously doesn't take place. If you have a good idea how well-produced music sounds, and how its tonal balance sounds, you can get a good idea if an amplifier is reproducing sounds correctly.

You don't need all the octaves to hear if the tonal balance is correct, so a good studio monitor will work, and I choose the LS3/5a for my monitoring. The room acoustics must be good, and sound absorption must be similar to the end-users abode.

Then it's just a case of listening to as much music as you can - quite a rich catalogue of genres - and all of them must provide some satisfaction, and not glower with glare at you, unless recorded particularly badly.

The other thing I do is keep the amplifier circuit switched on over long periods of time because the sound changes if something isn't right in a design. I believe most designers who might be employed 9 - 5 will not have the "luxury" and might therefore be blinded to a particular design discipline. This is quite dangerous to their reputations in my opinion.

I have listened to circuits which sound incredibly good, only to find they sound as distorted as hell after a few days of being switched on. Returning them to the test apparatus, nothing is amiss, but to the ear it obviously is.

Therefore observation is the most important of all, no matter what the know-it-all’s say or believe.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 17 Jan 2020 at 7:17pm
Originally posted by peterb peterb wrote:

Would an SMD version of the 1N4148 stuck with a thermal conductive glue make it a bit easier?


If you can attach two wires to it, all well and good. Personally I can't. Wink
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