A couple of problems with messing with R4 - the offset will change, and there is a triggering of bandwidth changes - which may call for a cap bypass of R4 to maintain stability.
It would be better to manipulate R3, the offset would change, true. A cap would be needed across R4, which would stabilize the feedback loop. See any of the microphone pre-amps where this is done - Sam Groner's SG-SOA1 doc, John Hardy's 990 pdf, Jensen Transformer's JE-16 data sheet.
It would be better to manipulate R3, the offset would change, true. A cap would be needed across R4, which would stabilize the feedback loop. See any of the microphone pre-amps where this is done - Sam Groner's SG-SOA1 doc, John Hardy's 990 pdf, Jensen Transformer's JE-16 data sheet.
Yeah, there are lots of good reasons why the traditional attenuator is used rather over a variable gain amplifier, despite the disadvantages of the former.
In fact the deeper you dig into it, the less attractive it becomes.
However if all you need is a trim over a narrow range, rather than full adjustment, building out the existing gain switch (R3) to multiple settings (or a potentiometer) will do the job just fine.
In fact the deeper you dig into it, the less attractive it becomes.
However if all you need is a trim over a narrow range, rather than full adjustment, building out the existing gain switch (R3) to multiple settings (or a potentiometer) will do the job just fine.
Indeed, this simple hack has changed my perception of the ad774 in this application. Prior that with ad744 on-board sound wasn't engaging even though highs were better defined than opa134's.
Notwithstanding opa134 is a really good choice for the default opamp. Works pretty good in my setup. It's cheap, very musical and lets listener be charmed by well mastered recordings (especially binaural ones).
I've never really found an application where I liked the OPA134/2134. I've listened to it with and without some type of buffer after it.
Of course, everyone's perception of what sounds "good" varies from person to person.
I prefer bipolar op-amps...despite their higher input bias currents and DC offsets.
As stated earlier here, you have to be very careful when trying bipolar op-amps in Richard's Sapphire amps.
Of course, everyone's perception of what sounds "good" varies from person to person.
I prefer bipolar op-amps...despite their higher input bias currents and DC offsets.
As stated earlier here, you have to be very careful when trying bipolar op-amps in Richard's Sapphire amps.
one of the biggest problems i have with most of the opamps i have tried is that there is a 'squeaky (like rubbing a rim of a glass with a wet finger)' sound to the T's and a slight sibilance to the S's. the only one that does not do this as much is the OPA551. if i could cross the AD744 with the OPA551 i would be a happy chappy. the OPA134 is superb apart from the slightly tubby bass which most OPAXXX opamps seem to have but this may be due me having DT770 ear pads on the DT880's but when i put the DT880 pads back on everything sound too thin for my liking. the AD opamps seem to me to have a tighter bass.
i'm going to order a few more opamps to try out as i like messing around and you never know i might just stumble on one i really like the sound of.
i'm not trying to create the 'i'm there' sound, if i wanted that i'll go to a gig or pick up the guitar. i'm just wanting to get rid of the slight niggles such as the T's, S's (yes i know its sometimes the mike they are using in the studio) and slight tearing now and again.
anyway V3 ordered so i will see how that goes. if its a big jump like going from the 14s to the v2 i'll be happy.
what would be really nice is to have the facility to switch between a few opamps on the fly.
i'm going to order a few more opamps to try out as i like messing around and you never know i might just stumble on one i really like the sound of.
i'm not trying to create the 'i'm there' sound, if i wanted that i'll go to a gig or pick up the guitar. i'm just wanting to get rid of the slight niggles such as the T's, S's (yes i know its sometimes the mike they are using in the studio) and slight tearing now and again.
anyway V3 ordered so i will see how that goes. if its a big jump like going from the 14s to the v2 i'll be happy.
what would be really nice is to have the facility to switch between a few opamps on the fly.
messing around again last night this time with coupling caps. all i can say is if people cant tell the difference between different caps then they need to get the wax out their ears.
removed the .68 Rike S caps and put in .68 Mundorf Evo oil and the results were staggeringly different, gone was the tubby bass and hello airiness only problem was that it also removed a huge portion of the low end. the good news is that instruments now had a lot of decay to them like symbol crashes lasted a lot longer and easier to pick out different instruments. bad news is that all the above is at expense of smoothness and everything had a rough edge to it also the snare drum for some reason is brought a lot more forward and is very prominent same with vocals and its like the singer has taken about 6 steps forward in the soundscape but is now playing the snare as well. bass guitar notes were more recognisable along with more bite to the leading edge but they had no weight to them. the kick drum was so recessed that it was almost removed from the mix. stringed instruments were sublime if a tad higher than they should be along with piano and wood/brass wind. soundscape was very 3d.
step up the 2.2 Evo oil, and hello low end as well so much so i had to remove the DT770 ears pads and put the 880 ones back on. everything now is like the .68 Evo oil but with more low end, but again all at the expense of a slight roughness but not as much as with the .68. the Evo oil 2.2 dont have as much time on them as the .68 do so they might get a bit better so i think i'll leave them in for a while.
all the above was with the OPA134 on the boards. the difference was a lot more noticeable swapping the caps than it is swapping the opamps. the AD744 now sounded very brittle and rough when i tried them.
the Rike S caps are very good but i just wanted to mess about and i'll leave the 2.2 Evo Oil in for a while to get accustomed to them then swap the Rikes back in.
removed the .68 Rike S caps and put in .68 Mundorf Evo oil and the results were staggeringly different, gone was the tubby bass and hello airiness only problem was that it also removed a huge portion of the low end. the good news is that instruments now had a lot of decay to them like symbol crashes lasted a lot longer and easier to pick out different instruments. bad news is that all the above is at expense of smoothness and everything had a rough edge to it also the snare drum for some reason is brought a lot more forward and is very prominent same with vocals and its like the singer has taken about 6 steps forward in the soundscape but is now playing the snare as well. bass guitar notes were more recognisable along with more bite to the leading edge but they had no weight to them. the kick drum was so recessed that it was almost removed from the mix. stringed instruments were sublime if a tad higher than they should be along with piano and wood/brass wind. soundscape was very 3d.
step up the 2.2 Evo oil, and hello low end as well so much so i had to remove the DT770 ears pads and put the 880 ones back on. everything now is like the .68 Evo oil but with more low end, but again all at the expense of a slight roughness but not as much as with the .68. the Evo oil 2.2 dont have as much time on them as the .68 do so they might get a bit better so i think i'll leave them in for a while.
all the above was with the OPA134 on the boards. the difference was a lot more noticeable swapping the caps than it is swapping the opamps. the AD744 now sounded very brittle and rough when i tried them.
the Rike S caps are very good but i just wanted to mess about and i'll leave the 2.2 Evo Oil in for a while to get accustomed to them then swap the Rikes back in.
I got my 3.1n boards in the mail this week. Great quality boards and looks like it will be a really fun build!
I've been reading up on rectifier snubbers in http://www.diyaudio.com/forums/powe...sformer-snubber-using-quasimodo-test-jig.html and am looking to make a PCB for a through hole toroidal (2x115V PCB Mounting Toroidal Power Transformers from Nuvotem Talema), rectifiers, and potentially CRC snubbers that will be calculated using the Quasimodo. I like the idea of keeping everything clean and PCB mounted.
Do you think that a snubber would be a good addition to the power supply circuit? Wholly unnecessary or worth a try (and a fun way for me to work on my PCB layout skills)?
I've been reading up on rectifier snubbers in http://www.diyaudio.com/forums/powe...sformer-snubber-using-quasimodo-test-jig.html and am looking to make a PCB for a through hole toroidal (2x115V PCB Mounting Toroidal Power Transformers from Nuvotem Talema), rectifiers, and potentially CRC snubbers that will be calculated using the Quasimodo. I like the idea of keeping everything clean and PCB mounted.
Do you think that a snubber would be a good addition to the power supply circuit? Wholly unnecessary or worth a try (and a fun way for me to work on my PCB layout skills)?
Snubbers affect the ringing created by the diodes, so how effective they are perceived to be depends a lot on what diodes are being used. It also depends on how and how much of that ringing is translated to the output. Obviously the circuit PSRR comes into play, but also things like the capacitance and inductance of the filter capacitors and associated wiring, as the capacitor ringing may or may not induce oscillations in the rectified power supply.
To be honest, I never heard much of a difference and do not bother with it myself.
To be honest, I never heard much of a difference and do not bother with it myself.
That being said, I would still recommend bypassing each rectifier diodes with several 10 nF each as the device may otherwise pollute the mains in the longwave and mediumwave regions, up to lower shortwave, inducing annoying hum in AM stations (a diode is a nonlinearity after all, and will tend to create intermod between real stations and mains). Equipment designers used to do this all the time, otherwise tuners would have been a bit unhappy. Alternatively, use super fast recovery diodes.
just got round to trying this out a few days ago. bent pin 6 out the way then jumped pin 5 to 6 under the board.
all i can say is a BIG thank you. smooth and open but some might call it 'dry'. vocals are superb and if i were to criticise it for anything i would say there is a slight dampening to the extreme highs. another added bonus (for me) is the left to right seems to be drawn more into the middle and forward which makes for a better listening due to not having 'blobs' in each ear when panning, more of a middle of the head soundscape this in turn has made the headphones less obvious. there is also a slight loss of depth but i can forgo that for the sheer listening pleasure.
not tried any classical music yet.
finally finished the V3 boards 
and a little mod to the opamp sockets for using the AD744
now just to plug them in and see if they blow up
An externally hosted image should be here but it was not working when we last tested it.
and a little mod to the opamp sockets for using the AD744
An externally hosted image should be here but it was not working when we last tested it.
now just to plug them in and see if they blow up
wired up and plugged in.
only been on for less than an hour and it sounds lovely. i have the AD744's in the sockets as that is what i have been listening to on the V2 boards but i'll put the OPA134's in in a few days once everything has settled down.
apart from the mod to the opamp socket i used Omite little rebel CF instead of Kamaya CC and Dale CMF55 in the gain resistor network. but everything else is what Richard sent me. i also de-fluxed the boards down this time.. lol
only been on for less than an hour and it sounds lovely. i have the AD744's in the sockets as that is what i have been listening to on the V2 boards but i'll put the OPA134's in in a few days once everything has settled down.
apart from the mod to the opamp socket i used Omite little rebel CF instead of Kamaya CC and Dale CMF55 in the gain resistor network. but everything else is what Richard sent me. i also de-fluxed the boards down this time.. lol