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Old 29th August 2003, 10:15 PM   #1
jkeny is offline jkeny  Ireland
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Default TDA1545 question

I built the OPA660 IV circuit for TDA1545a as per Kuei's schematic a while back and found output from TDA1545 was too low for the circuit to work. Resistors to vref pin 7 were 11K & 11K with 3.3V vref taken after first 11K res.

I thought I had blown my 1545 Dac as when I tested output from dac it was only 0.7V accross 1K IV resistor. I ordered more and tested them before installation in circuit (using 1K IV res as before). Same result 0.7V.

I have confirmed this with another inmate who opened his working Marantz CD4000 and rerouted output through 1k IV res - same result.

What gives? Is the 1545a current output at digital silence not 2.5mA ? Bias current in datasheet states (Ibias) typically 0.7mA and full scale current (Ifs) 2mA (when using 11K + 11K res as above). Where does Kuei get the 2.5mA current at digital silence from?

Can anybody help? I would love to hear this circuit working as I intend to compare it to R Broertjes jfet looped cascode output stage which sounds excellent.

John
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Old 30th August 2003, 01:00 PM   #2
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Default Re: TDA1545 question

Konnichiwa,

Quote:
Originally posted by jkeny
I thought I had blown my 1545 Dac as when I tested output from dac it was only 0.7V accross 1K IV resistor. I ordered more and tested them before installation in circuit (using 1K IV res as before). Same result 0.7V.
Did you confirm that you had actually applied a digital signal that equates digital silenece, so that the DAC was not sitting merely at the the lowest current (0mA) instead of Digital silence (1mA)?

Because with only 0.7V across 1k this indicates 0.7mA current which would not leave enough room for the DAC to swing +/-1mA....

Did you play music and get ANY signal (on the 'scope) for the 1k I/V resistor? Did you try raising this resistor to 2k?

Sayonara
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Old 30th August 2003, 02:21 PM   #3
jkeny is offline jkeny  Ireland
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Kuei,
thank you for replying and sorry for taking up your time on such trivial issues -

?
Quote:
Did you confirm that you had actually applied a digital signal that equates digital silenece, so that the DAC was not sitting merely at the the lowest current (0mA) instead of Digital silence (1mA)?
yes I have played cd and signal accross 1K IV res is still 0.7V but faint music is present. Is digital silence equivalent to gap between tracks?

Should Ibias go up to 1.4V when 11K + 11K resistors on Vref are used
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