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Old 26th January 2004, 04:51 AM   #1
tcpip is offline tcpip  India
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Default Simple discrete unity gain buffer

My friend Angshu designed this buffer for me, when I wanted a very simple unity-gain discrete buffer. His simulation runs show THD at -107dB with a load impedance of 10K. And PSRR seems to be -49.5dB on the positive rail and -53dB on the negative.

What do you guys think of this circuit? Angshu is quite a veteran of transistor based circuits, so I believe this will work pretty much as the sims show; neither of us has breadboarded it yet. I'll probably be using something like a BC550 instead of the BC337 he's used. I guess any high-gain low noise transistor will do just fine. C3 and C1 are for DC blocking. The output will have a DC offset of about -0.7V without C1. This is not a problem for me, because I always put input caps in the signal paths of my power amps anyway. And yes, I'll put a decent regulated PSU for this buffer; just bridge plus caps may not do, since PSRR is way worse than opamps.

Quite often, line-level preamps don't need any gain. Putting a volume control pot before this buffer should be enough to build an active preamp which can drive fairly long interconnects (10 feet?). Many of us need simple buffer-based preamps for many projects. And I need longish interconnects because I am building active loudspeakers, where each speaker has an active xo plus a set of power amps, in a chassis placed adjacent to each speaker. Thus, my interconnects need to go from my preamp (in a central rack) to each speaker. I am not sure passive preamps are a good idea with such long interconnects.

What do you guys think of this buffer instead of an opamp-based buffer? What should I be careful about?

Thanks,
Tarun

PS: I also posted a query about a remote controlled volume control design in another thread. Comments please??
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Old 26th January 2004, 05:21 AM   #2
tcpip is offline tcpip  India
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Sorry guys, the last time, I couldn't get the image to fit into the 100KB limit without shrinking. That was because it was true-colour. I've now quantised the colours to 20 colours, and I can retain the original full size in 73KB.
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Old 26th January 2004, 06:46 AM   #3
AKSA is offline AKSA  Australia
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Looks good to me.

Perhaps a 470R resistor between the base of the current sink and the LED, increase the cap to 100uF, but nothing more.

Purists might comment adversely on two coupling caps; you could run it without the input cap if the source was safe, but good caps will see you right.

Should easily produce 25Vpp with a Zout around 26/Ic; not clear what current you are running as the LED voltage is not clear. I would assume around 4-6mA.

Current delivery to load will be high on positive half cycle, but limited by the CCS on negative half cycle. There will be highish H2 at high loading.

Cheers,

Hugh
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Old 26th January 2004, 07:00 AM   #4
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I agree with Hugh; since you are running two power supplies, the input coupling cap is superfluous and can be left out. On the other hand, if you think you need it because you may have sources with DC, then you could leave out one of the power supplies without adverse effects.

Beware: simulation THD results very seldom have any relationship to real-world results, they are a trend indicator at best. Noting that this really is an emitter follower, how does the sim results change with say 1000 to 3000pF parallel to the load to take care of the cable cap?

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Old 26th January 2004, 10:05 AM   #5
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Quote:
Originally posted by AKSA
Perhaps a 470R resistor between the base of the current sink and the LED, increase the cap to 100uF, but nothing more.
Will do. Noted.
Quote:
Purists might comment adversely on two coupling caps; you could run it without the input cap if the source was safe, but good caps will see you right.
Yes, I'll leave it out, almost certainly. I just kept it in the schematic because that's how my friend had simulated it.
Quote:
Should easily produce 25Vpp with a Zout around 26/Ic; not clear what current you are running as the LED voltage is not clear. I would assume around 4-6mA.
I'm stuck on this question; Angshu is out of town. Once he's back, I'll ask him to fill in the blanks. But I think the current will probably be around the range you quoted.
Quote:
Current delivery to load will be high on positive half cycle, but limited by the CCS on negative half cycle. There will be highish H2 at high loading.
What I'm really curious to know is how it'll behave with a load impedance made up of, say, 20K resistive plus the impedance of the 10-feet-long interconnects. I will probably be using RG58 or RG59 coax cable used for Arcnet or 10base5 Ethernet as the interconnect. What will that load be like? Does this qualify as "high loading"?

[QUOTE]Originally posted by janneman
I agree with Hugh; since you are running two power supplies, the input coupling cap is superfluous and can be left out.[/quote
Yes, I'll do that.
Quote:
Beware: simulation THD results very seldom have any relationship to real-world results, they are a trend indicator at best. Noting that this really is an emitter follower, how does the sim results change with say 1000 to 3000pF parallel to the load to take care of the cable cap?
Angshu has the simulator. Will try that as soon as he returns, and will post results. What L and C values should one take to model longish interconnects? Will I actually have C as high as 3nF if I use video-grade coax? Wow. If these are the figures, how do they carry Ethernet at 10MHz over 100 metres?
And overall, what do you guys think about the quality of this as an output bufffer, compared to a decent opamp (say, NE5532/OPA2134) used as unity-gain inverting amp? I mean, what, if any, will be the audible differences?

Thanks for the help, Hugh and Jan.
Tarun
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Old 26th January 2004, 10:17 AM   #6
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You could also consider a diamond buffer of some kind. We have discussed it here quite much lately. A diamond buffer is real easy to get going and you wont need a output cap.
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Old 26th January 2004, 10:19 AM   #7
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Quote:
Originally posted by tcpip
[snip]. What L and C values should one take to model longish interconnects? Will I actually have C as high as 3nF if I use video-grade coax? Wow. If these are the figures, how do they carry Ethernet at 10MHz over 100 metres?
And overall, what do you guys think about the quality of this as an output bufffer, compared to a decent opamp (say, NE5532/OPA2134) used as unity-gain inverting amp? I mean, what, if any, will be the audible differences?Tarun

Tarun,

I had some interconnect that were 300pF/m, not that much really. For 10m you get 3000pF. I don't know about the video stuff, maybe you can measure a length with a cap meter? Or contruct a filter with a series R and the cable, then find the half-voltage freq, R = 1/(2. pie.f.C), and calculate the C.

Difference with an opamp: I don't know about the audilel difference, because that depends on a lot of other factors in your system. The measurable differences will be very large.

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Old 26th January 2004, 11:58 AM   #8
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Default Cable capacitance

Capacitance of RG-59 is typically 22pF per foot and general purpose shielded cable ( local brands ) for microphones is about 40 pF per foot.
I think Tarun's cable is 10 feet not 10 meters. So at 220pF or 400pF it is pretty OK.

All RG-59's are not the same. Especially at DC. The core conductor can be single core copper or copper coated steel or multistrand copper.
The braid can be stranded aluminum or copper , single layer or multiple layers or aluminum foil plus strands . Quite different from each other.
Additionally the insulator varies from foam to solid and the insulator material varies with the brand. So one cannot use the term RG-59 by itself. You will have to mention the type and brand name.
The same applies for other shielded cables also. Can lead to A LOT of experimenting.

As a line level interconnect the cables with low capacitance will avoid early rolloff . I am not quite sure how the dc characteristic will affect this. As a line level interconnect this should not affect the low end. But this will surely be important if used as a speaker interconnect - as some fans do. LF performance could drop . I see people mention this effect in their set up's .

With a Zout of 1K ohm ( yes ,that's very high ) and a load of 10 K Ohm ( 20 K volume pot ) and cable capacitance of 400pF , the roll off is about -0.1dB at 70 Khz. With a Z out of 100 ohms it will be -0.1db at 700Khz. With a 10 K volume pot the frequencies will be half of the above.

I think Tarun's interlink is quite OK on paper. BUT he will have to try several types to determine how they 'sound'. That appears to be a grey area and needs practical tests - no (known ?) sim can give the correct answer. Don't forget to use good connectors !
Cheers.
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Old 26th January 2004, 12:01 PM   #9
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Ashok, thanks for the numbers. I'll try to remember them!

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Old 26th January 2004, 01:34 PM   #10
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Default Re: Cable capacitance

Thanks, Ashok. Good to see you in this thread.

Quote:
Originally posted by ashok
Capacitance of RG-59 is typically 22pF per foot and general purpose shielded cable ( local brands ) for microphones is about 40 pF per foot.
I think Tarun's cable is 10 feet not 10 meters. So at 220pF or 400pF it is pretty OK.
Correct. 15 feet (5 metres) max.

Quote:
All RG-59's are not the same.
Thanks for the enlightening inputs. And I am picking up something which has stranded copper core and braided (no foil) shield. Don't know about the insulator material. Any brand of cable I should look for?

Quote:
As a line level interconnect the cables with low capacitance will avoid early rolloff.
That's what I was thinking. Call me ignorant, but I was under the impression that cable-to-cable differences in RG58/59 would be insignificant for audio frequencies, and the capacitance issues would be inaudible if I drove them using a low-Zout buffer stage. The volume control and other controls, if at all, would come before the buffer, not after. So, at least in my case, the cable will see a low source impedance, and feed directly to a power amp of, say 10KOhms Zin. In case the inputs of multiple power amps are connected in parallel and fed from the same interconnect (active speakers, one amp per driver for multiple midbass units), then the Zin of the load may be lower.

So, driving a cable of 15 feet from a low-Zout buffer of the type I've drawn, and feeding a load of 10K or so, should be safe at audio frequencies. Or so I thought!

Quote:
I think Tarun's interlink is quite OK on paper. BUT he will have to try several types to determine how they 'sound'. That appears to be a grey area and needs practical tests - no (known ?) sim can give the correct answer. Don't forget to use good connectors!
Thanks. Will try. Regarding RCA plugs, my current favourite are some connectors with the "Monster" label, hefty and very tight-fitting. I'll continue to use those till someone teaches me better.

Tarun
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