DIY Audio Projects Forum

i researched a bit on the net and some say that there will be a 3db drop in the output signal of the dac....
ultimately it can go wrong if the input impedance of the MCA is too low.


regarding the fostex fe206e that I got especially for the MCA, can this little amp be modified not to send anything above 2 or 3khz to the fostex?

I'm not sure on what you mean nor do I know about dac splitting, but you should wire the two amp inputs in parallel. In theory it should not distort when you wire input of amps in parallel.

The impedance is 8ohms and with calculation.
https://www.electronicproducts.com/RC_F ... lator.aspx
With a 10uF the cutt off is about 1.99khz
6.8uF on onput of mca is about 2.92khz cutt off. (this is high pass)

http://sim.okawa-denshi.jp/en/CRtool.php
I would suggest that you try 82ohms in series with input then a 1uF to ground for a 1.94KHZ low pass filter
82 and 0.68uf for a cut off of 2.85khz
Keep in mind your amp has input resistance, therefore the resistance of filter must be made small,otherwise loading effect on output changes the freq response.
Keep in mind the MCA is a inverting amp, and it may sound weird when your sub amp is non inverting but your mid highs are inverted.

I would suggest that you low pass the forstex full range at about 6-8khz before you put tweeters, full range should generally have better vocal midrange soudn quality than, midrange ,tweeters.
From my epxrience you can boost the highs a bit of any speaker by connecting a zobel for it. Something to look at. My 4inch fullrange lacking of highs had got a good fix by having a zobel.
http://diyaudioprojects.com/Technical/Speaker-Zobel/

After some research i found out that the best dac for signal splitting is one with a tube output stage. but i also have found that some put a single jfet instead of an opamp at the output of a dac.
is it possible to use a single jfet or tube at the output of a dac which would also at the same time supply biast current for the mca?

You can consider using the supply from your mca and making a small regulator with lm317T, no heat sinks required since small load. The voltage you need to drop also can be calculated ask me for further details.

You can of course try runing it direct, or also try a rc filter, This means putting a 220-4.7k ohm resistor in seriers with the supply from mca, at the other end of 4.7k connecting a resonable size cap 470uF+ cap from 4.7k to ground, this acts as a simple regulator that is good for running small loads. This is how many valve amp guys get smmoth supply.

Just finished my MCA build. While setting the bias I measured the DC offset and am getting a whopping 14V at the speaker terminals. Any ideas? It`s such a simple circuit maybe I read the schematic wrong. I have 28 volts as the power supply

With a 28V power supply the speaker output point should be 14V which is 1/2 supply voltage. This allows for the same amount of positive wave and negative wave to be reproduced. The speaker itself does not see the 14VDC directly due to the Output Capacitor which blocks this voltage from getting to the spkr but, allows the AC signal threw. So if you measure from gnd to the point between the mos and the 317 it should be 14V with a 28V supply. This point would be before the output cap. But when you measure at the spkr to gnd (after the cap) it should be zero volts DC until a signal is output to the spkr which would be AC.

This is correct of course.

Did you have a 8ohm dummy load connected after the output caps? What do you measure across the dummy load? The positive terminal of the cap should be connected to the collector.

_________________
Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
Website: retro-thermionic

I do have the collector to the + of the output cap. The 14v is measured from - binding to + binding post.
Will acquire some high wattage 8 ohm resistors and see what I get

The output may float high with no load. If no music is playing a 1W 8 to 15ohm resistor should be ok.

Also have a source, e.g. CD player connected to the input RCAs. This will stop noise getting into the amp.

_________________
Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
Website: retro-thermionic

with an 8 ohm 10w resistor across the binding post I get 0.0mv
thanks so much for the help, onto stabilizing the bias
cheers!