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Informative questions before starting β24 build

beta24

Re: Informative questions before starting β24 build

Postby amb » July 30th, 2017, 11:41 am

I don't think you need to obsess too much about this. Just choose whichever that will be most convenient to get and use, taking into consideration the chassis fitment and cost of course.
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Re: Informative questions before starting β24 build

Postby cobretti » July 31st, 2017, 1:56 pm

Advantage of separate transformers is that, there is no capacitive couplings through transformer windings.
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Re: Informative questions before starting β24 build

Postby Pieterh » August 27th, 2017, 6:23 am

Hi Ti,

I was looking for the C1/C2 1.5pF (C0G /NP0) 100V multilayer ceramic capacitors for the Zobel network. How about these? Are these right?

http://nl.mouser.com/ProductDetail/TDK/ ... NuRQ%3d%3d

Mouser no. 810-FG18C0G2A1R5CNT6

These have 2,5mm lead space (in stock). The 5mm LS would be better but are not in stock.
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Re: Informative questions before starting β24 build

Postby amb » August 27th, 2017, 1:25 pm

The 1.5pF caps are not part of the zobel network. They are compensation caps in the negative feedback loops. The capacitor you linked are fine for this purpose.

If you really do mean the zobel network, they are Rz and Cz on the schematic and parts list, to be mounted on the speaker binding posts (not on the PCB). The values are 10Ω and 47nF.
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Re: Informative questions before starting β24 build

Postby Pieterh » August 28th, 2017, 7:43 am

I have misunderstood the scheme of the input/output/NFB/zobel. I thought the triangle represented the b24 pcb and the rest belong to the zobel network. Now I understand that only Rz 10 Ohms and Cz 47nF make up the zobel. The other parts are already on the b24 pcb.....

Then, can I ask; why do the c1 and c2 values differ between b24 bom (1.2 pF) and the input..zobel drawing (1.5pF)? Anyway, I have already soldered the 1.2 pF smd's..... But if 1.2 or 1.5 pF doesn't matter, others may use this new 1.5 through hole capacitor. More easy to solder......
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Re: Informative questions before starting β24 build

Postby amb » August 28th, 2017, 8:36 pm

When I was developing the β24, 1.5pF came out as the best value during simulation. But under actual bench test I determined that it's ok to drop it to 1.2pF and still maintain good stability. The simulation did not take into account the small amount of leakage capacitance on the PCB itself. I set up the parts list based on this finding but neglected to change the schematic diagram.

Nevertheless the difference of 0.3pF is really small. You can use either value and it will work fine.

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Re: Informative questions before starting β24 build

Postby mpa2 » August 29th, 2017, 1:10 am

Ti, what is actually the benefit of such high bandwidth (up to 1 MHz) in audio amps?
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Re: Informative questions before starting β24 build

Postby amb » August 29th, 2017, 1:50 am

Very good question, and there are two schools of thought on this subject.

1. The amplifier only needs as much bandwidth as it is necessary for audio frequencies, all else is junk and should be eliminated.
2. A wide bandwidth is beneficial because any frequency rolloff (and the corresponding phase shift in the time domain) are going to be well outside of the audio band.

I tend to go with the second one. I am a big fan of Erno Borbely (famous engineer behind many outstanding and notable amplifiers). Almost all of his designs have very wide bandwidth. I also admire his virtually universal use of complimentary differential topologies, which is an important inspiration for the α20, β22 and β24.

Some might argue that certain sources have undesirable ultrasonic noise that the amplifier could filter out, but I believe in designing well-behaved sources that do not produce such junk. I don't think it's the job of a power amplifier (or for that matter, the preamp) to limit the bandwidth. Others say that too much bandwidth makes the amplifier susceptible to RFI and may in some cases turn them into radio receivers. Well, that is certainly true in poorly implemented amplifiers, but my experience has been good in that regard. None of my high bandwidth amps have done anything ill in that manner.

It's not that we have any useful audio content in the ultrasonic frequencies, but high bandwidth and fast slew rate go hand in hand. You cannnot have one without the other. A fast amplifier ensures that it cannot be a limiting factor on the signal being amplified. Too slow an amplifier could cause SID (slew-induced distortion) and TIM (transient intermodulation distortion).

Our ears perceive volume changes in a logarithmic manner, and it also perceive frequency changes the same way. This is why frequency response graphs are commonly logarithmic on both the X and Y axis. And it's why an octave is every doubling of the frequency and a decade is every 10x. If you look at a logarithmic frequency response graph that extends to 1MHz, you will find that 1MHz is not as "far away" from 20KHz as you might think. For example, to go from 100Hz to 1KHz (one decade) is perceived similarly as from 10KHz to 100KHz (one decade). Another decade later you're at 1MHz!

The phase shifts that accompany the frequency rolloff will begin much earlier within the audio band. If an amplifier has low bandwidth, even if it exceeds 20KHz comfortably, the phase shift will encroach into the treble region.

It is harder to design a wideband amplifier, and to make it stable. But I think the extra effort in optimizing the bandwidth, slew rate, and square wave response is justified.

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Re: Informative questions before starting β24 build

Postby Pieterh » September 7th, 2017, 12:08 pm

Can I double check the positioning of the JFETs? For the B24 we need to look at the right hand side picture on the B22 instruction page, right?
Then the JFET shapes on the PCB by the silkscreen are all incorrect. But the S-D-G pads/markings on the PCB are in the correct order, right?
In your schematic of the B24, the Source Pin of the JFETs are marked with a dot, right? I added some pictures where i didnt solder those JFETS yet. I got it right? Thanks!
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Re: Informative questions before starting β24 build

Postby Pieterh » September 7th, 2017, 12:10 pm

and a fourth photo
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