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Post by Deleted on Dec 6, 2007 21:18:02 GMT
2knowtalent
What rebuilt analogue stage ? Where are the transformers located, in between stages or at the output ? Do you have a schematic ? In general, it is never desireable to have more than several milliVolts DC out of a component. With a lower voltage amplifier, such a DC offset will mean that the output stage has a reduced linear voltage swing capability as well.
SandyK
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Post by 2knowtalent on Dec 7, 2007 15:02:11 GMT
2knowtalent What rebuilt analogue stage ? Where are the transformers located, in between stages or at the output ? Do you have a schematic ? In general, it is never desireable to have more than several milliVolts DC out of a component. With a lower voltage amplifier, such a DC offset will mean that the output stage has a reduced linear voltage swing capability as well. SandyK I'm having my Arcam CD23 modded and they're installing coupling transformers directly off the opamps that convert the Ring DAC output from current to voltage (or is that the other way around???). This way all the caps and such downstream of the opamps...all the way to the RCA outputs are removed (as far as I know). The stock specs call the RCA output at 2.3V while adding the transformers will increase this value to 4V. I recall talking to Mike about the mV bleed that you mention when I was trying to decide whether or not to install the input filter caps when I modded my X-Can V3. I put them in. What I'm clearly not understanding is when the manufacturer calls 2.3V at the RCA outs, how does that relate to the mV bleed that you mention??? Again, I admit complete ingnorance to what this all means but the guy who is doing the work has been at this for years and talks like it's no big deal. When I asked him about it he said he could rewind the transformers to 1:1 if I wanted but he's never had any problems in the past... He just modded a guy's X-Can V2 and he's now running a similarly modded Pioneer CDP into it with no problems. I called a tech guy at the MF rebuild center in North Carolina and he said the only issue he could foresee was less volume required to achieve same output levels. Am I walking toward a potential tiger trap on this? |
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Post by Deleted on Dec 7, 2007 20:46:59 GMT
"the manufacturer calls 2.3V at the RCA outs"
2knowtalent
I am having a great deal of difficulty accepting that statement !
Normally an opamp is biased so that it gives close to ZERO DC out. Some manufacturers however, do not always follow this convention. This low DC out is also usually a result of using both -VE and +VE DC supply rails. Again, some manufacturers use only a +VE supply rail, in which case there is usually somewhere around 1/2 rail voltage at the output, and a capacitor is necessary. In the case of some DACs they have a current output . The TDA1541 has around 1.8mA, and this causes a DC offset, which can either be nulled by special circuitry, or an output capacitor used.
Leo is better equipped to give the definitive answer with this particular problem, as he is much more across DACs and current mods in that area.
Nevertheless, a DC output at the actual RCA sockets is NEVER desireable. I presume there will be an output capacitor installed prior to the RCA sockets.
SandyK
BTW
I know Rick's feeling on the subject, but as far as I am concerned, any "expert" who finds it necessary to insert a frigging coupling transformer in such an area doesn't have a full grasp of the subject ! It may sound good to some people, but the potential quality is degraded in many areas.
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