You could just run a pair of LM675 set for 20dB of gain. Then you probably wouldn't even need the opamp buffer.
Long-term follow up. I went through several dual op-amps for a 5x gain stage and then started on single op-amps. LT1028 was detailed but the treble was rather brittle, AD845 was better but treble was a little un-smooth. Maybe oscillation problems or something. The best turned out to be a Signetics NE5534 - smooth and quite natural sounding. I got some PCBs from Hong Kong which worked nicely.
So here we are at what looks like a stage I can live with. Schematic attached.
So here we are at what looks like a stage I can live with. Schematic attached.
Attachments
Thanks for sharing Andy. I received this board you linked to earlier...who knows...going to tinker with it soon.
NE5532 Dynamic Microphone Stereo Preamp Amplifier Board DIY DC9-24V AC8-16V | eBay
Do you have a link to the other pcb's you got from HK?
NE5532 Dynamic Microphone Stereo Preamp Amplifier Board DIY DC9-24V AC8-16V | eBay
Do you have a link to the other pcb's you got from HK?
That is kind of a lot of capacitance at the input. You're driving this with the transformer output of a DAC?
The DAC has a NE5532 output. What sort of capacitor value did you have in mind, and what's the theory behind sizing this capacitor?
It is a short to ground at high frequencies. Whether that value makes sense or not depends a lot on what's driving the op-amp.
I'm assuming that the purpose is just a low pass filter to keep high frequency garbage out.
If I flip through my copy of Douglas Self's Small Signal Audio Design I see that he typically uses 100R in place of R4 and 100pF in place of C2.
If I flip through my copy of Douglas Self's Small Signal Audio Design I see that he typically uses 100R in place of R4 and 100pF in place of C2.
Thanks, Heath. 100R and 100pf is a 6db rolloff of 15,900Khz which is pretty high. 470 and 1.5nF is 226Khz which at least is doing something.
But don't forget that the output impedance of the previous stage also factors in as audiowize points out. If it has significant output impedance, your rolloff will shift down and start causing measurable rolloff in the audio band.
I'd use the 100R and 100pF on the non-inverting input and then use the cap in parallel to the feedback resistor to define a lower rolloff for the stage if desired. That one will function the same no matter what piece of equipment you connect to input or output.
This design gets an A+ from me at least (especially since I did something very similar already)
The only thing about the 801a is how freaking FRAGILE the filaments are. Thoriated Tungsten is such a brittle thing.
I don't like buying them online since nearly half of them are DOA from knocks and bumps....
Pretty white glow, if your postal service is not butter-fingers.
In the last decade I bought a few dozen of TT filament tubes on ebay. Non of them arrived in broken state or with torn filament.
A few 841 tubes -especially ultra old globe ones- deceased during use, but it's "normal" due to the internal structure.
The TT filament at each heating/cooling down period (at about 600-700 Celsius) has grain reorientation (Miller-Larson effect), so this momentary plastic state can cause the filament wire to grow in length, and therefore become thinner.
Sooner or later each TT filament run out, and thinned wire would breaks.
801a (even more graphite anode 801) has more robust filament wire. With careful heating up (I use its only with Rod C. regulators) can be used for a very long time.
A few 841 tubes -especially ultra old globe ones- deceased during use, but it's "normal" due to the internal structure.
The TT filament at each heating/cooling down period (at about 600-700 Celsius) has grain reorientation (Miller-Larson effect), so this momentary plastic state can cause the filament wire to grow in length, and therefore become thinner.
Sooner or later each TT filament run out, and thinned wire would breaks.
801a (even more graphite anode 801) has more robust filament wire. With careful heating up (I use its only with Rod C. regulators) can be used for a very long time.
Hi Juanitox,
Have you tried the 2p29l pentode driver yet?
I just received some 2p29l and this idea interests me.
Regards,
John
hi, yes i have tried them in triode mode and don't prefer them to the 26 more delicate and quieter , those high fast pentode tube are difficult to keep quiet.
but since this time i have change my 26 /300B to a EL3N /AD1 amp , and those old IDHT power pentode sounds pretty good as driver too 🙂
but since this time i have change my 26 /300B to a EL3N /AD1 amp , and those old IDHT power pentode sounds pretty good as driver too 🙂
DHT Rob has a SE amp design also. Interesting that DHT fans are using the EL3N. He has triode curves too.
Hi Juanitox,
Thanks for the response!
I'm thinking over various ways to drive an SE 300b. I have OPTs for them. Just need to build an amp.
May I ask a few questions?
1- Just to clarify, did you try the 2p29L in pentode mode?
2- Are you running the EL3N in triode or pentode mode?
3- Are you running the EL3N/300b as just 2 stages?
3b- do you have a schematic you would like to share?
4- Have you compared the EL3N to EL84s or 6V6s run the same way? (the later would be a bit easier to find over here.)
Thanks!
Regards,
John
Thanks for the response!
I'm thinking over various ways to drive an SE 300b. I have OPTs for them. Just need to build an amp.
May I ask a few questions?
1- Just to clarify, did you try the 2p29L in pentode mode?
2- Are you running the EL3N in triode or pentode mode?
3- Are you running the EL3N/300b as just 2 stages?
3b- do you have a schematic you would like to share?
4- Have you compared the EL3N to EL84s or 6V6s run the same way? (the later would be a bit easier to find over here.)
Thanks!
Regards,
John
I wish I had this experience. Seriously, 50% I bought arrived doa so I just gave up for the past two years. But 801a driving 300b with IT is very nice indeed. If I build another one, I will post a pic.
1 no just triode
2 triode
3 now i'm running EL3n AD1 but my dac is a 5V output and i don't need full gain
3b no i don't follow particular schematic my amps are in constant change 😀
4 i have not compared EL3n and 6V6 in this amp but 6V6 is the US successor of EL3n all you need to know is they are power tubes too so you need a low impedance driving source for their miller effect but they will move easilly your 300B and gives you a fuller sounds than small signal triode.
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