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Old 13th April 2003, 02:52 PM   #1
dre is offline dre
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Default offset probelm in bugle phono preamp

I'm finishing my Bugle phono preamp for MC elements so I using the resistor vlaues between brackets (see schematic: http://www.hagtech.com/images/bugleschem.gif ) I left out R12/R16 and R30/R34.

At the inputs of the first opamps I measure a 15mV offset. This results in an output voltage of U++ (15Volt) at the output of the second opamp.

I think the reason for this is the 3uA bias current of the AD797 I'm using and the 47500Ohm input ressistance.

If I reduce R17 to 1k the ofset voltage is reduced proportionally.

I wonder what is the max dc ofset allowed for an MC element. What is the max Input resistance of phono pre-pre's for MC-elements?

Any more advices?
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Old 13th April 2003, 03:07 PM   #2
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What DC offset do you measure on the output of U1A? I'd guess around 160 mV, right? With teh gain around U2A, that should result in about 2.3 volt offset at U2A output, so I reckon you have a problem in the ciruit around U2A for starters. Doublecheck your wiring, and you may have invert and non invert inputs swapped on U2A.

To reduce the input DC offset from being amplified, put a capacitor in series with R5, 100 uF should do it.

To prevent DC offset into the cartridge, you'd have to put a series cap with the input, 10 uF or so.

I don't see R30/R34 in the circuit....

Lukas
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Old 13th April 2003, 03:25 PM   #3
dre is offline dre
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On the output of U1A I measure around 500 mV. Which is what I expect ; 15 mV *(13000/393) = 15mV *33 = 500mV (about)

500mV * 33 is Vcc

Sorry, my fault, R30/R34 is on the full schematic, which consists of both channels, left and right.

I do not quite understand how the capacitor will reduce the offset, as reducing R17 reduces the offset.

Are you sure that putting a fat capacitor at the input of the phono prepre is okay by the cartridge? I've never seen any prepre with a capacitor in the input.
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Old 13th April 2003, 04:20 PM   #4
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LL - OK right, you changed the down resistor value from teh 1K3 to 390 ohm? Yea, that will make the input stgae much higher....

I do not quite understand how the capacitor will reduce the offset, as reducing R17 reduces the offset.

LL - It will make DC gain unity, while maintaing AC gain, so DC offset at U1A out should be teh same as at the input. For a 392 ohm down resistor you'd have to use 470 uF or so.

Are you sure that putting a fat capacitor at the input of the phono prepre is okay by the cartridge? I've never seen any prepre with a capacitor in the input.

LL - LOL you just haven't seen enough circuits then.......
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Old 13th April 2003, 07:23 PM   #5
dre is offline dre
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Quote:
Originally posted by LukasLouw
LL - It will make DC gain unity, while maintaing AC gain, so DC offset at U1A out should be teh same as at the input. For a 392 ohm down resistor you'd have to use 470 uF or so.

LL - LOL you just haven't seen enough circuits then....... [/B]
Ooops I shoold have seen that I'll get unity gain with a capacitor at R5. Good point!

Maybe I've not seen enough circuits. In the circuit I did see ther were either no capacitors at all or some selectable capacitors between line en ground. In that case the user could match the input capacitance to the cartridge in use.

Nevertheless I'll consider it as an option to have a big decoupling capacitor in the input. I'll experiment with it. See what it does soundwise.

Can you tell me what the max allowed offset would be as far as the MC-cartridge is concerned? I would not want to damage my cartridge for the sake of experimenting.

If possible I'd like to find the solution in a differnt opamp, as the PCB is already made and the parts are soldered on them. This kind of makes it undesired to put in a capacitor at R5. After all the original design which is commercially available does it without extra capacitors.
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Old 13th April 2003, 07:41 PM   #6
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Default input-cap

Hi!
To safe-guard your cartridge, an input-cap from 1-10 uF can be used, as in Linsley-Hood RIAA. (1uF).

http://users.classicfm.net/williamshart/phono.htm

Arne K
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