Mod
One design goal of the project was to test Xfeed. Done : I never use it !
We can easily mod Ampcasq in order to :
As a positive side effect, the input impedance won't be, at least within audio range, anymore reactive and frequency dependant.
I'll do the calculation tomorrow.
One design goal of the project was to test Xfeed. Done : I never use it !
We can easily mod Ampcasq in order to :
- remove Xfeed,
- use the Xfeed switch as a gain selector.
As a positive side effect, the input impedance won't be, at least within audio range, anymore reactive and frequency dependant.
I'll do the calculation tomorrow.
As I understood the Xfeed, it's mostly of use when listening to recordings from the early stereo days; when an instrument would sometimes be mastered to a single channel. Modern recordings with proper mastering should not benefit from Xfeed at all. So much for the theory. I still need to order parts, so I didn't test it myself 
One channel of my O2 has a little bit more gain than the other. Definitely inaudible to me, but my ancient, analogue HM312 with cheap china-probes will easily expose that 20mV output difference at 5V peak-to-peak
Not audible is basically good enough, but you can use a dual channel oscilloscope for that
One channel of my O2 has a little bit more gain than the other. Definitely inaudible to me, but my ancient, analogue HM312 with cheap china-probes will easily expose that 20mV output difference at 5V peak-to-peak
To avoid having a lot of attenuation before having a lot gain with OPA1642 amplifier, it's better to lower the gain of the amplifier thus lowering the need for attenuation.
Current gain of AmpCasq is (switch in central position) 10.3dB which is the result of the attenuation mainly provided by R1 and RV1A, and of the the amplification set by R18, R13.
changing R18 & R19 to 1K reduce the gain of the 1642 amp
changing R1 & R4 to 4.7K reduce the attenuation, thus with switch in central position gain of Ampcasq will be 9.4dB, at bit less than 10.3 but still ok for your T1.
changing R6 & R8 to 6.8K will give total gain of 6dB when switch in corresponding position
changing R7 & R9 to 3K will give total gain of 3dB when switch in corresponding position.
With those mods the switch will allow you to choose between +3, +6 or +9dB gain.
Equivalent resistor seen by the C5/C6 cap will evolve between 1.5K and 3.2K, cut off frequency of the filter will be between 500KHz and 1MHz, a bit high but ok.
If you want to give a try before ordering anything else, you can change R7&R9 to 10K (you sould have some 10K left on your previous order), total gain will be changed to +6.4dB.
Current gain of AmpCasq is (switch in central position) 10.3dB which is the result of the attenuation mainly provided by R1 and RV1A, and of the the amplification set by R18, R13.
changing R18 & R19 to 1K reduce the gain of the 1642 amp
changing R1 & R4 to 4.7K reduce the attenuation, thus with switch in central position gain of Ampcasq will be 9.4dB, at bit less than 10.3 but still ok for your T1.
changing R6 & R8 to 6.8K will give total gain of 6dB when switch in corresponding position
changing R7 & R9 to 3K will give total gain of 3dB when switch in corresponding position.
With those mods the switch will allow you to choose between +3, +6 or +9dB gain.
Equivalent resistor seen by the C5/C6 cap will evolve between 1.5K and 3.2K, cut off frequency of the filter will be between 500KHz and 1MHz, a bit high but ok.
If you want to give a try before ordering anything else, you can change R7&R9 to 10K (you sould have some 10K left on your previous order), total gain will be changed to +6.4dB.
Well, I got all the resistors from Digikey...fast service and they dont charge you a farm out west for shipping!! The ship small orders via first class post....
I removed the caps, shorted, removed all the resistors and replaced with the above values and it now works with three selectable gain settings via the switch.
Better than before to me.
Now all I need is that blue led later this month, and will install and paint the enclosure!
oh also will test again when the 12vdc linear adapter shows up as well.
Alex
I removed the caps, shorted, removed all the resistors and replaced with the above values and it now works with three selectable gain settings via the switch.
Better than before to me.
Now all I need is that blue led later this month, and will install and paint the enclosure!
oh also will test again when the 12vdc linear adapter shows up as well.
Alex
Alex, I don't know about "out west" shipping charges, but I find DigiKey is just too damn high on most thru-hole components I purchase.
Plus, they charge sales tax for out of state purchases!
I can get the same parts for less from Mouser and shipping is only $4.99 from TX to TN and NO sales tax. Mouser rules!
Now that you've got your parts, let's bust out the whiskey and the soldering iron and get that OPA1622 project done!
Well there ya go...interesting. I will have to re-look those shipping costs etc...I dont want to sound like shill for one company or another....The fast service to me maybe because they are located closer to me than mouser in north dakota...it takes longer for stuff to get here than digikey...
Whisky..no...but some good craft beer, gose, lambic or sour!! LOL
Well its really all done except for the blue LED which isnt needed and I need spray paint the front and back panels from all my drilling and scraping !!
I will put up some pix as soon as i get this done...led is late JUNE!! #$%^&* talk about disappointment!
Alex
(Note I am working on the "Crocodile" amp now that Sergey has in a different thread...the parts density there are crazy close)....
Whisky..no...but some good craft beer, gose, lambic or sour!! LOL
Well its really all done except for the blue LED which isnt needed and I need spray paint the front and back panels from all my drilling and scraping !!
I will put up some pix as soon as i get this done...led is late JUNE!! #$%^&* talk about disappointment!
Alex
(Note I am working on the "Crocodile" amp now that Sergey has in a different thread...the parts density there are crazy close)....
I noticed that you're working on that crazy 'ole Crocodile amp also.
The LM6172 has a reputation for being a "nasty" op-amp that likes to oscillate. Hmmm...kinda sounds like the AD797...LOL.
Hopefully, the PCB you're using is designed to stop any oscillations "dead in their tracks".
Your patience with working with all those SMDs has to be greater than Job and much greater than mine.
Keep up the good work!
Well, I hope so as well...the main reason I am building the crocodile is to "hear" how this amp with some really good objective specifications "sounds" or doesn't sound...it also brings back all those design memories from NWAVguy...in his efforts to build an amp with low cost, optimal part placement etc...to get great results...
I don't want to hi-jack Chris's thread so we can chat further if desired over in the Croc thread...
Alex
I don't want to hi-jack Chris's thread so we can chat further if desired over in the Croc thread...
Alex
Hi Alex,
I've lend my AmpCasq to CaféNoir a DIYA member who works close to my office, we should have some measurements soon.
Chris
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Boards
Alex,
I've read that your blue led has come, you now could hear the blue sound ! no doubt the crocodile will have a green one !.
Do you want a board to practice or one to do a second AmpCasq ?
I can send you both, as soon as will be back from travelling, next saturday morning.
Chris
Alex,
I've read that your blue led has come, you now could hear the blue sound ! no doubt the crocodile will have a green one !.
Do you want a board to practice or one to do a second AmpCasq ?
I can send you both, as soon as will be back from travelling, next saturday morning.
Chris
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