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Old 15th October 2007, 09:57 PM   #1
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Default Oh no! Not another discrete IV converter

Hi All,
I have finally managed to build a discrete IV stage for my TDA1541A based DAC.
I've only just got it working, with my 'experimenting' PSU (only regulated with 7815/7915) and have only listened to it briefly through headphones.
However, on first impressions it sounds alright actually. I think there is a lot of detail so far. I am looking forward to feeding it from my DAC PSU, which is regulated with lm3x7, and then TL431. Then I'll be able to compare it to my humble LM6172.

The circuit is a duplicate of the one posted (obviously for stereo), but the voltage bias (generated by the 20k trimmer) is shared by both channels. I assume this is good practice, as it sets the gain for both stages. Or should I have two biases? One per channel?
Adjusting the bias is subtle, and at extremes causes distortion, so I suspect I will need to adjust it using a test CD and oscilloscope to ensure distortion is reduced/eliminated as best as possible.

I am also hoping to get this running on a split supply, rather than 0-15v, as the DC offset is quite large.

Also, I suspect I will follow it with a current sourced CFP based emitter follower, and I'll add a voltage offset to reduce the DC offset to zero.

Cheers,
Phil
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Old 16th October 2007, 09:01 AM   #2
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Had another listen this morning.
It does sound slightly brighter than my opamp stage (from memory), but I do suspect some of that might be down to having only heard this through my Grados (which are a little bright in nature compared to my speakers).
There does seem to be lots of detail, and I do get the impression that its unlikely to be fatiguing.
I think there is a lot of potential for this stage when it is running off my DAC PSU.
Tonight I'll check the bias and thus output with my scope and have a proper listen on my LS3s tonight.

I'm just really gutted I have only MP3s on a PC and cheapish headphones at work.

But overall, very pleased
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Old 17th October 2007, 12:10 AM   #3
Ipanema is offline Ipanema  Malaysia
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Hi philpoole,

Have you tried to compared your I/V with Pass D1?

Regards.
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Old 17th October 2007, 03:39 AM   #4
Pars is offline Pars  United States
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6K is probably a bit high for a TDA1541A on the I/V resistor... usually you see around 1.5K. Also, you might want to use a 1st order filter, say around 1.8nf or so in parallel with the I/V resistor (off the top of my head, and with the 1.5K).
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Old 17th October 2007, 09:22 AM   #5
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Thank goodness, some feedback. I need it!

Ipanema,
This is my first adventure into discrete IV (as you'll discover as you read the rest of this post ), so alas I haven't compared to the Pass D1.

Pars,
Is the 6k resistor the IV resistor? I thought it was used to provide a voltage bias for the common base amplifier - although I'd be lying if I said that I completely understood this circuit.
I use this bias for both IV converters (left and right) and I still have a stereo image, so I suspect this isn't an IV resistor.
I do suspect 6k is a bit high though...

Last night I connected it up to an oscilloscope and noticed that the lower half of a full scale sine wave could be distorted (smaller in magnitude than the upper half) if not very carefully adjusted. I suspect this is due to the amount of current into the base - which led me to my first (probably of many) mistake. I think the potentiometer should be a variable resistor, which could then be tweaked to allow more current into the base. Since then, I've noticed that is what is done in other IV converters. I'll change that and see what happens.

I finally managed to listen last night and I was amazed by how it sounded. I can't say if its better (I suspect it is), but its sounds totally different to my old opamp IV converter. Its typical to expect incremental improvements when changing things, but this sounds completely different. Almost like Monty Python.
Perhaps its because its open loop, perhaps its because its class A, or both.
It must be an improvement, because there are lots of things I've never heard before I am now hearing.
I do like it, just need to get used to it.

Anyway, I need to change the bias circuit. I agree 6k might be too high (although I did see one circuit with 100k and a 20k variable resistor!), and I need to make the pot a variable resistor to increase current.
Also, I need to find a definitive text on common base amplifiers. Horowitz and Hill on briefly touch on the subject. None of my other electronics books really discuss it. I've found a few articles on the web (wikipedia has a reasonable description). I suppose I haven't found anything quite applicable to how it works as an IV converter.
That's why I decided to build one and learn through investigation.

All I currently (forgive the pun) think I know is that its essentially a voltage amplifier with a very low input impedance, which the DAC likes (as its a current sink), and the gain is set by the voltage bias (although it would make more sense to me if it was a current bias) into the base.
If anyone has the correct description, or a link to it, I'd be very grateful.
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Old 17th October 2007, 10:21 AM   #6
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I guess this is the best I've seen so far. Must sit down and read it properly...

http://users.ece.gatech.edu/~mleach/.../bjt/cbamp.pdf
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Old 17th October 2007, 12:14 PM   #7
x-pro is offline x-pro  United Kingdom
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There is a thread on similar circuits and I've posted there my schematics from 1994:

http://www.diyaudio.com/forums/showt...203#post229203

As you may see it looks very similar to yours and indeed it sounded very nice. However I've incorporated some LP filtering - perhaps it may help to cure the brightness you're experiencing.

Cheers

Alex
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Old 17th October 2007, 12:33 PM   #8
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Hi Alex,

Thanks for the link. Hopefully it will be useful reading.

What kind of regulation did you use for your IV converter? With the low PSRR of these kinds of circuit, I'm interested on what others have settled for.

I really want to improve my understanding of this circuit so I can understand how component changes affect the system.

I do actually have a 3rd order passive filter after this stage as the DAC is 4x oversampling.

I think it initially sounded a bit bright because I hadn't adjusted the bias with an oscilloscope to minimise distortion, and I was listening using my Grados - which tend to be a bit bright (I tend to listen to new DIY kit with rubbish sony headphones first, then the Grados, then the amp and speakers - just in case something gets fried).
Listening through my LS3/5As, the harshness is gone thankfully.

I think at the moment my bias circuit is incorrect (I reckon its limiting the current into the base) and hopefully fixing that'll help a lot.

Cheers,
Phil
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Old 17th October 2007, 01:08 PM   #9
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Quote:
Originally posted by philpoole
[snip]Pars,
Is the 6k resistor the IV resistor? I thought it was used to provide a voltage bias for the common base amplifier - although I'd be lying if I said that I completely understood this circuit.
I use this bias for both IV converters (left and right) and I still have a stereo image, so I suspect this isn't an IV resistor.
I do suspect 6k is a bit high though...[snip]

I think he means R3, not R6...

Jan Didden
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Old 17th October 2007, 01:21 PM   #10
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by philpoole
What kind of regulation did you use for your IV converter? With the low PSRR of these kinds of circuit, I'm interested on what others have settled for.
I've used at the time just a plain stabilsed supply on LM317.

Quote:
Originally posted by philpoole
I think at the moment my bias circuit is incorrect (I reckon its limiting the current into the base) and hopefully fixing that'll help a lot.
To start with you may try adding another 10-22 uF capacitor between the ground and the base of Q3. At the moment the base is looking into the pot and can see up to 5K there. It will transfer directly into 25-50 additional Ohms of the input impedance.

Cheers

Alex
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