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Old 10th February 2002, 09:08 PM   #1
Freddie is offline Freddie  Sweden
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Exclamation DAC output stage

What do you think about this?
I'm going to use this to drive my Aleph amplifer directly without any preamp. I will probably use two PCM1716 DACs, one in normal and one in inverted mode to have a fully balanced DAC. But perhaps I will use something better like CS43122, I haven't made up my mind yet.

Comments, Suggestions?
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Old 16th February 2002, 10:19 AM   #2
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The in- and output- buffer will work very well .. but I would choose to discard the cap's... you couls easely make it DC-coupled with vey low offset.... I'm very intersted in the way you will use the volume controll... .. Do you have a remote controll schematic.. ? or any schematic for the controll of the switches.. ? How are you gonna deal with the transient upon switching?

thijs
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Old 16th February 2002, 02:12 PM   #3
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Default DAC output stage

Ah..... the Borbely Jfet buffer. I was getting ready to try that circuit this week with my passive preamp. The circuit has a DC offset since the dynamic current source conected to the negative supply is operating at a diiferent DC Vgs than the follower. You could put the same resistor after the follower and match the FETs for DC coupling.

I don't know about splitting the K389s since the two fets have the same substate. Use two and paralell the two fets in one for the follower in in the other for the current sink. This circuit needs very clean power supplies also. Don't take the caps out, they provide the feed back. I think this would work fine with the DC offset issue fixed.

H.H.
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Old 16th February 2002, 02:15 PM   #4
Freddie is offline Freddie  Sweden
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Default Thanks for the reply.

The relays will be controlled either by a Altera EPM7064 or by a microcontroller (PIC16F84) which counts pulses and senses rotation of an optical encoder. I haven't decided yet. I will buy an Altera EPM7064 next week, If I find it usefull I will use it, otherwise I will go for the PIC16F84. I have never tried the EPM7064 before...

So if I need an attenuation of -48dB only one resistor will be in the signal way. An attenuation of 60dB would mean that two resistors are in the signal way. Max attenuation is 94,5dB (six resistor in the signal way).

As for the transient when switching, I think the relays are fast enough so there won't be any audible click/pop when changing volume setting. I use relays for switching between different sources on my preamp, and there is no audible pop or click.

As for the remote controll, I will use Holtek HT12-D and HT12-E chips to make a simple transmitter/receiver.
More info about the Holtek chip can be found here.
http://www.holtek.com/

Here's a schematic for a receiver/transmitter using the above mentioned ICs.
http://www.rentron.com/project03.htm

I'm a little unsure about the the crossover frequency I should use for filtering. The filter in the schematic is a Bessel 60kHz 12dB/octave. Perhaps I should use a lower crossover frequency?

/Freddie
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Old 16th February 2002, 02:29 PM   #5
Freddie is offline Freddie  Sweden
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HarryHaller,

Yes it's the Borbely Jfet buffer

And the volume controlled is actually based on the one LcAudio used in there Sidewinder preamp.

I'm thinking of using a normal Jfet buffer with a constant current source, something like figure 15c in the article written by Borbely as this will have close to DC offset if the Jfet's are matched.
But I haven't made up my mind yet.

/Freddie
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Old 16th February 2002, 04:44 PM   #6
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Default Borbely FET follower

Use the Borbely circuit. Put the same value resistor after the the follower fet for DC offset The current sink fet should have the same Idss as the folower. Match them. The resistor is not in the Borbely circuit since it is AC coupled and Erno was trying to save money on those expesive resistors IMHO. Put a 1 Meg resistor to ground from the gate of the follower fets. DC coupled circuits really don't like floating inputs. Your woofers will thank me. 1000 ohm resistors as gate stopers will help the sound. Scott Nixon of Anodyne fame taught me how to voice tube circuits by changing the VALUE of the grid stopper resistors in tube circuits. RF stability and bandwidth limiting are very important to get audio circuit to sound right. Look for RC fiters on inputs and zoble networks on outputs to make a come back. They were there on the old stuff and are even a better idea today with all the RF pollution.

H.H.
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Old 16th February 2002, 07:08 PM   #7
Freddie is offline Freddie  Sweden
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Default Borbely follower

HarryHaller,

Does it look OK?
I simulated it in CircuitMaker, and it showed a DC offset of about 1mV.

/Freddie
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Old 16th February 2002, 10:50 PM   #8
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Default Borbely buffer

Put a 1K in series with the gate on the lower fet also. The actual
offset will depend on how close the Saturated drain current is for the two fets. When you build it or layout the board try to keep the lead from the 1K resistor as short as you can make it.

H.H.
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Old 9th July 2002, 02:51 AM   #9
dorkus is offline dorkus  United States
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Default how does it sound

hi freddy,

did you build your circuit? any impressions on sound quality?

cheers,
dorkus
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Old 9th July 2002, 06:10 AM   #10
LBHajdu is offline LBHajdu  United States
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Default reed relays

Going back to the idea of using relays for the volume attenuator, has anybody ever tried using reed relays for this? I would like to try making a preamp with reed relays however I am worried about contact bounsc and mercury reed relays are too pricey for the number of them that I would need.

As for the output section, I am curious to know if a passives preamp could be used with this Dac. Does anybody know what the voltage swing of this circuit is going to be at the output?

Please keep us informed of your progress I would like to know how this one works out.
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