SX-Amp and NX-Amp

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#2,124
also according to this table http://users.tpg.com.au/gerskine/greg/vas transistors.htm

i think we can use others ... also just for knowledge

1-i don't know how to choose the VAS transistors ... i mean this based only on the supply rail voltage and matched Hfe
2 - the lower capacitance Cre is better or the higher capacitance is better
3 - also the frequency do i need this frequency ?they all ranging between 60 and 400 Mhz there is no signal carrier so only sonic frequency is needed 20hz to 20 khz .
4 - the power rating should be high (15 to 20 watts) to be more rigid and thermaly stable or it should be low( 2 to 10 watts).

sorry for long questions

M.Hanafy
 
#2,125
Some circuits demand that the Cob be very low to achieve design performance.
Some circuits use the Cob to roll off the treble/HF/VHF to make the circuit stable.
You would need to know which version of the circuit that you are experimenting with.

Obviously Vce0 and Pmax must allow for the full range of operating voltages and currents.

Most circuits use only one VAS transistor (sometimes called TIS).
This topology does not require hFE matching.

fT can be anywhere that the designer chooses.
Going lower can increase the phase delay through the stage and can make the circuit less stable.
The opposite also applies. Higher fT can make the circuit more stable because there is a lower phase delay in the stage. Again you need to know which applies in your circuit if you intend changing for no good reason.
Changing the current through the VAS/TIS will often change the transistor's speed. This can make the circuit more, or less stable. Inadvertant current change happens when the Builder chooses a different rail voltage and/or allows the mains voltage to vary over the full range allowed by mains tolerance.
 
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#2,129
@Andrew.... thank you ,,,this means that all depends on the design ... there is general rule to apply here .

i think i miss a lot of reading and info ..... i just build and follow posts till my amp work if it was not...

any one suggest me a good easy ref to read about audio amp circuits basics
 
#2,132
SX listening experience.. I have been pretty busy lately so just running the SX from the variac for 21.5 volt dc supplies. I have run it closer to 25v and it does not get too hot but further work is to be done to resolve that. I am using ripped CD FLAC files and Arcam D33 DAC. I have my old floorstanders and very lucky to have the recently acquired Zingali Overture as part of a deal.


Earlier on nycavsr2000 asked for some subjective views. After a couple more evenings listening to it I would offer my personal observations; It is not fair to compare with the old JLH monoblocks because their capacitors are tired. The JLH were built with the best audio components I could get and possessed a lovely tone. They were easy on the ear and very informative in the mid range. I do not feel inclined to rebuild the JLH as am delighted with the SX. I am thoroughly enjoying using it. For the last four years I have been listening to high end Stax ES headphones and Kevin Gilmore amplifiers so am familiar with very revealing systems.
The SX has very low noise, and has great speed, clarity and definition. It has a neutral frequency response from very low to very high. The separation of musical instruments and the ability to follow separate strands in the performance is excellent. The music is free of congestion or muddle. Bass quality and definition is impressive. It is revealing of source material.
As I mentioned previously I used BC550C and 560C and the e grade driver pairs from Joshvi. All were matched within a few percent. The offset at cold is a few mV and it remains a few mV for a heatsink temp of 44C after several hours. For ease of construction I used the case from a link in this thread ;
https://www.aliexpress.com/item/431...ifier-Enclosure-Black-Version/1864830450.html
So to sum up; a great amplifier, perfectly suited to driving reasonably efficient speakers.
Results I am getting are everything I hoped they would be.. Thank you very much Bonsai!
 
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#2,134
gcl said:
SX listening experience..
Earlier on nycavsr2000 asked for some subjective views.
Results I am getting are everything I hoped they would be.. Thank you very much Bonsai!
Click to expand...

I am tickled pink that you took my request seriously! Kudos, and thank you for your impressions.

The SX is definitely on my list of amplifiers to build as I am curious about the CFA topology. I just have to get cracking on ordering the semi's and learn how to match them. If anybody has links for matching BJT's, please post here. It seems that the diamond buffer transistors, and the TIS transistors should be matched. How about the outputs, Q1,Q2,Q4 and Q5? And Q3? It seems lonely.

Best,

Anand.
 
#2,135
I just have to get cracking on ordering the semi's and learn how to match them. If anybody has links for matching BJT's, please post here. It seems that the diamond buffer transistors, and the TIS transistors should be matched. How about the outputs, Q1,Q2,Q4 and Q5? And Q3? It seems lonely.
Best,
Anand.[/QUOTE]

Often it is the practical tips on how to go about these builds which help builders achieve success. I hope you won’t mind if I say how I tested the transistors for the SX. I decided to use all BC550c and 560C so could buy a reasonable number to select from. I have two devices for identifying transistor types and leads and measuring gain, the Peak Atlas DCA55 and Duoyi transistor tester. If you are doing a lot of transistor amp work the Duoyi is very useful indeed. Although both are useful.


DUOYI Digital Transistor Parameter Tester DY294 | eBay


For the SX I bought 100 of each BC550C and 560c (about £4/100) and sorted them into gain groups on the Duoyi. I then picked the gain groups of hfe around 400-450 from both npn and pnp transistors. Then retested those and at the same time measure Vbe with DVM.
Result; 24 pnp transistors with gain from 440-460 and Vbe 0.651-0.655
Similar number of npn with gain 447-465 and Vbe 0.651-0.654
Having written these test results down it is fairly easy to select close matches.
You can type them into Excel and simply sort for whatever you want.
The Duoyi will also test Zener voltages so at least you can get some idea where you are with those. Example, I bought 10 of 78- TZX10C 2% and sorted for the four closest.
 
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#2,136
Wow. Fantastic info. Thanks a bunch, and that DUOYI machine is rather inexpensive. I was looking online and it seemed that as long as I had a regulated PS handy, I could build a circuit PTP, but your link is much easier and less headache.

I do have hfe available on my multimeters, but given that the DUOYI is specifically built for this purpose, I'll take your advice to heart.

How are you measuring Vbe using a multimeter?

Thanks again for the advice,

Anand.
 
#2,137
Put the transistor into the Duoyi and read/write down the hfe, immediately pick up your multimeter probes set to read1 or 2v and hold them across the base and emitter leads, reading the voltage. You have to be quick and consistent with your method so that the transistor doesn't heat up and values change. While on the subject, I use a stabilised 6v dc supply for the transistor tester. The batteries run down and you get a slowly changing result when doing a large batch, this is a waste of time!

I should add about the driver transistors, I bought 20 pairs of the e grade from Joshvi and did the same thing. I can build another SX with the unused ones now!

I found out of the several power transistors I bought from Mouser they were all close enough.
 
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#2,138
Bill of Materials for Ovation sx-Amplifier

Hello,

I wondered if anyone has a working link for the Bill of Materials for this amp please?

The link on the website does not seem to be working, and, so far, my searches for one have been fruitless :/

Many thanks 🙂
 
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