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A thread for Entech fans

May 25, 2007 at 4:33 AM Post #31 of 50
Quote:

Originally Posted by Faust2D /img/forum/go_quote.gif
Does anyone know if LME49720 works in Entech 205.2 out put stage?


I believe this is the new part number for LM4562 which I now use in my Entech 205.2. I had some problems with oscillation and had to insert 47-ohm resistors between the buffer amp outputs and the muting relay contacts. The result was very good. IMO this is the best amp to use. You may want to try OPA2107 as it works well without needing other modifications.
 
May 25, 2007 at 5:09 PM Post #33 of 50
One symptom of oscillation is overheating of the opamp. This will also consume extra power and degrade the power supply quality.

A less obvious symptom of instability is transient distortion. This will probably be audible.

The major factors are capacitive loading of the op-amp outputs and phase delay through the transistor buffer (100% feedback at high frequencies is taken from the buffer output). Some amps have a high tolerance for capacitive loading (AD in particular design this way) and others simply don't have enough bandwidth to cause problems. Amps with larger gain-bandwidth will be more likely to have stability problems.

The stock OPA2134 has about 8 MHz bandwidth and tolerates at least 100pf output loading (see attachment for gain = +1).

The LM4653 has about 55 MHz bandwidth and is rated for 16% overshoot with 100pf load (no details on closed-loop gain).

In the stock 205.3 the output jacks are indirectly connected to the buffer output through the relay contacts. Since there is no isolating impedance, capacitive loading will affect the phase margin of the feedback (the 203.2 has no transistor buffers and isolates the mute circuit and output jacks with a 600 ohm resistor).

Theory is theory. Sound is what matters. If you like the sound of your DAC then just enjoy it.
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May 25, 2007 at 5:58 PM Post #35 of 50
Yes, all the dual DIP-8 amps work more or less well, OP275 (fet-bipolar), OPA2107 (di-fet), OPA2604 (fet), OPA2227 (bipolar).

If you are well-funded consider AD-825 (fast fet) or OPA627 (fast di-fet). These will require a converter as they are single amps.
 
May 25, 2007 at 7:50 PM Post #37 of 50
I expect all of the first four as I tested them myself. I have not tried OPA627 (expensive) but I had problems with AD825 (40 MHz) and LM6172 (100 MHz). It will depend somewhat on the cables used.

If you are handy at soldering you can try the mods I made for stability.

IMG%20014a.jpg
 
May 25, 2007 at 8:28 PM Post #38 of 50
This is awesome info, thanks so much for helping me out. I am not very handy at soldering, but I have someone who will help me out. I am thinking about making an Uber Entech by making a new case with a nice power supply and upgrading a few things here and there. When I have it ready I will post info here.
 
May 30, 2007 at 2:14 AM Post #39 of 50
anyone know if it's okay to leave these entechs on all day?

i have mine connected to my desktop that happens to be on at all times.

i always hear the dac click in and out to get the data locked even when someone on my msn list pops up, it works fine, but is this safe?

thanks
 
May 30, 2007 at 6:29 AM Post #41 of 50
Quote:

Originally Posted by sonic_rage /img/forum/go_quote.gif
anyone know if it's okay to leave these entechs on all day?

i have mine connected to my desktop that happens to be on at all times.

i always hear the dac click in and out to get the data locked even when someone on my msn list pops up, it works fine, but is this safe?

thanks


I would think leaving it on 24/7 is a lot safer than repeated power up/down cycles. Electronics generally prefer a steady state, as opposed to random fluctuation. I leave mine on all the time with no problems.
 
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