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Old 20th March 2010, 08:43 PM   #1011
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Quote:
Originally Posted by godfrey View Post
Hi Carlos

Perhaps you could put a resistor at the collector of Q6, to drop some voltage and share the power?

Regards - Godfrey
Yes, during the development of the SiliconChip ULD Mk 2 (a blameless amp), they found Q6 would blow if the amp was overdriven. In their case they found a tight fitting RCA connector on the input disconnected the earth before the positive was causing the problem.

They fixed it by adding a 22K resistor in the position you suggested.

regards
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Old 20th March 2010, 08:44 PM   #1012
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Default Removed, after strong pression people did, the capacitor that loads VAS

The amplifier passed all torture tests....the room is a mess, wires all around, solder spots, parts replaced...dirty place.

I am here to inform you the Toroidal transformer could not resist the test..also it was presenting noises, maybe some internal bad solder, not a good contact, it was putting dirty in my scope and was driving me mad!.....now everything is fine.

Wave shape good.... sinus becoming triangle only above 150 kilohertz.... low frequency response fine...but really i have lost something i cannot define yet..quality losses when i reduced current to first VAS and the emitter resistor at second stage VAS....well.... at least was not destroyed.

I really think i have to offer something safe...and this is what i am doing...but really, to my own use, my home use...my sound amplifier, then i will remove the modifications and will go back to the Dx Blame ST...the stable one, without the capacitor (10pf) loading second VAS.

Sound is clearly superior to average amplifiers...but this way it is now... the last VAS modified, ..... seems something gone.... there's no more that feeling of transparency i felt before....maybe i am just deliration....not uncommon to do that...we are humans....a lot of modifications...so, i am expecting changes..maybe nothing has changed...i will try to check this doing comparison and calling friends to help me to judge.

The problem i am facing is...the most stable, the most safe one..seem not to be the best sounding one.

Dx Blame ES was unstable to high levels of high frequencies..but sounded better than this Dx Blame ST and better than Dx Blame ST with these VAS modifications.

Leds have introduced too much noisy (audible) and where not needed...thank you by the suggestion.

regards,

Carlos
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Old 20th March 2010, 08:50 PM   #1013
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Default The capacitor was supressed.

Was removed..will not return anymore.

say good bye for it!

regards,

Carlos
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Old 20th March 2010, 09:06 PM   #1014
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Default When we follow standard practices of design/creation of amplifiers

we have, as a result, a standard amplifier in sonics..this is what i am realising these last monthes.

The crazy Dx Blame ES..... that could not face high levels of high frequency in the input..sounded much better than this last one......i have to check this comparing next days.

If confirmed, then i will go back to the first one..and will take the risks to have it burned because strange sources of audio with high levels or treble.

I will go inside my own home..my own risks, but i do not suggest you to do the same....i accept to go fixing from time to time to listen that lovely sound.

But for forum folks, builders and friends, my obligation is to offer the most reliable one, the most simple one, with a good sound reproduction.

regards,

Carlos
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Old 20th March 2010, 09:45 PM   #1015
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Default Modifications..... the 15 ohms second VAS emitter resistance

depends to cut the trace to solder the new resistance...this is not a pretty thing to be done...as i could not have it burned during my tests with 44 volts.... i think it is safe the way it is (BD139, second VAS transistor)...so.... not needed to include the 15 ohms resistance, to avoid to cut the board trace.

I am feeling the bass stronger..the treble has lost some level, it is there, but a little bit shy compared to the previous amplifier...the Dx Blame ES.... this is not final conclusion.... friends will help me...i have my own prejudices, so, i need people more able to judge this..so, i will keep one channel without modifications and will switch from one and other to the people will evaluate....maybe next week.

Yellow marks shows parts modified...no copper track to be cutted..parts are just removed or substituted by others..so..board is keep the way it is.

The coil removal is effective to reduce a small noise.... alike overshot.....do not influence sonics...so...not really needed to move coil.... some shield may be a nice idea... the coil shown in the Todd Johnson's artistic view is better than my own.... i am showing mine....better to build alike Todd have painted in the beautifull image he made.

regards,

Carlos
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Last edited by destroyer X; 20th March 2010 at 09:51 PM.
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Old 20th March 2010, 10:01 PM   #1016
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Default No more modifications will be made..final schematic.

further modifications only if i found errors in the schematic..but update and upgrade are closed.

Now people can build and have a very safe amplifier..super tested...have suffered torture the amplifier.

regards,

Carlos
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Old 20th March 2010, 11:39 PM   #1017
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Default Interesting, a couple of small modifications...i am using, rigth now, to listen

The second VAS emitter resistance (15 ohms).

Interesting, funny, and almost unbeliavable, even to myself...but bass is huge...remembers me the Dx Amplifier.... the treble seems shy...better than the Dx amplifier, but not too much better.

Details..small details...interesting.... changes the sonic signature.

regards,

Carlos
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Old 21st March 2010, 12:26 AM   #1018
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i would try to put a small capacitor - like 470p or so - in parallel to the 15R emitter resistor at Q7, to see what it does to sonics.

This resistance allows the base of the VAS to "move" voltage wise. This movement will be "transmitted" to the VAS output at the high frequency end through the millercap C13, so that the gain of the VAS cannot (in an idealised modell) go below 1 (0dB), thus "imparing" the compensation.

The said cap parallel to the 15R will ensure that VAS gain is able to roll off below 0dB. (if properly chosen in value, of course.)

Now, not having this cap can be good or bad - depends on the speed of the transistors, to put it simple.
If you don't face problems with stability without it, it's fine.

But who knows what the sonics will do. (I don't, thats the why of this post, actually )

So, I think the best way to find out, what it does, was to try.
Maybe sonics improve.

Also feel free to experiment with the value of the cap. If increased, it will influence lower and lower frequencies (then the influence goes the other direction, by the way, increasing the gain) and maybe start to have a real influence on loopgain in the audible range, and maybe bringing up the beloved highs.

use a good linear dielektric such as c0g, np0 or film/foil

regards,
krachkiste
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Old 21st March 2010, 12:39 AM   #1019
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Default Thank you Krachkiste

I am giving a break for a while.....next week will give a try,

regards,

Carlos
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Old 21st March 2010, 04:29 AM   #1020
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Default That BD139 was burned, previously, but because my supply producing strange signals

driving the amplifier to oscilation... after supply replacement by a 36 volts unit... was impossible to burn the BD139.

When using 44 volts, i was very near the transistor limits considering the max VCE, load was difficult too, low impedance (3.5 ohms), inductive as my resistance is constructed coiled and have metal core inside, and also i had more than 2uf in parallel with the resistance.

The introduction of 15 ohms resistance, or lower value, changed the reproduction signature, also was needed to cut a copper track....you can avoid that if you want.

Also, when i had the second VAS burned, also the first VAS had gone too...so... they are direct coupled.

No more transistors burned after the increase the first VAS colector load resistance to 220 ohms, having other capacitors removed/substitute made simultaneously.

BD139 is a 100 volts transistor in some datasheets, but i can remember it was specified as 80 volts transistor too, in other papers, so i am not sure if it is a 100 volts or 80 volts...using the 44V supply, them the peak to peak is dangerous to this transistor...shorted base to emitter (maybe because first VAS) and also shorted colector to emitter (maybe excess of swing).

Friends will come to listen.... and i will produce a board without modifications, will use the previous schematic, the Dx Blame ES...and will avoid difficult loads or extreme loud reproduction entering distortion... this will keep the ES safe...as it can play this way without problems... the amplifier, the ES, was alergic to difficult loads and very high containt of high frequencies clipping the amplifier...... the other amplifier, the second channel, will be the last schematic, and i will remove the 15 ohms resistance, because this way, keeping the resistance, no need to even compare..the new schematic will loose very easy...no chance to comparison.

Interesting...just that resistance changed the amplifier...not more shy bass....now the treble is shy... the Dx amplifier, the standard one, was not sensitive to these changes, i have included current sink in the differential, tried CCS in the differential, this two stage VAS...and i could not have the same result i had this time...there's a difference i am still wondering where is it.

There's an audible ratio i think... when you increase one, the other became shym (perceptive bass and trebles reproduction)...and this happens because you have listened the inverted sittuation before...so... the conclusion is because our mind was poluted...we really need fresh, brand new, without prejudice, evaluators to test.

I will try that capacitor, the ancient one, used in parallel with the emitter resistance..the one works increasing gain for AC, because it is a short to audio frequencies....it was used down the sixties, but to increase low frequency reproduction...usually they were microfarads units...will try, but next week..... for DC we go protected..but for AC, and that's our oscilating case, the introduction of capacitor will let the stage under danger i think.

regards,

Carlos
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Last edited by destroyer X; 21st March 2010 at 04:33 AM.
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