If a voltage gain is absolutely necessary (e. g. in cases, where there is only a buffer stage with unity gain as power amplifier), then this is a very good idea.
In all the other cases (i. e. if the power amplifier uses voltage gain stage factors between 10 and 100) I would prefer discrete preamps with only one stage in the signal path (source follower e. g.) - for an example go to
http://www.passdiy.com/pdf/B1 Buffer Preamp.pdf
or for more output current by use of multi-ampping systems
http://www.audiodesignguide.com/my/Follower_99c.gif
(use an idle current in such cases between 100mA and 200mA)
If there is a short connection to power amp with high input impedance above 20 K-Ohm, then I would prefer a passive pre-amp with only volume control and source select rotary switch.
check out therefore this thread:
http://www.diyaudio.com/forums/soli...nny-giles-tkd-vishay-more-whats-your-fav.html
most guys like for preamps circuits like this:
http://www.diyaudio.com/forums/solid-state/156685-jfet-audio-buffer.html
but I am not a friend of such topologies for the use in normal line stages applications where low gain factors realized.
Last edited:
well, don't know about the sensing circuit, but i have a stereo LM1875
amp (based on datasheet circuit), but it looks to me that it does not boil at all.
mine is built with decent heatsink and runs very cool. I never
measured its temperature, but i'm sure it runs at around 40° C...
Unfortunately i'm not brave enough to test if we can increase its bias with
a resistor... (read: i'm not well aware of what will happen, so i don't want to risk to blow a beautiful sounding amp)
amp (based on datasheet circuit), but it looks to me that it does not boil at all.
mine is built with decent heatsink and runs very cool. I never
measured its temperature, but i'm sure it runs at around 40° C...
Unfortunately i'm not brave enough to test if we can increase its bias with
a resistor... (read: i'm not well aware of what will happen, so i don't want to risk to blow a beautiful sounding amp)
Last edited:
Until now I had assumed that the bias current in an opa output stage would be about half the total supply current. OPAQ's results seem to indicate otherwise.
Anyway, an easy way to increase an opamp class A output current is to follow it with a buffer like the OPA634 or LM49600. You can set the buffer bias current often up to 20mA which means that it is in class A up to full output in less than 1k.
You take the opamp feedback from the buffer output so the whole thing acts like a hefty class A output opamp.
jan didden
At the risk of getting poo-shot, the LME49600 is much better than the BUF634 with raised BW connection.
(a bag of BUF634 by the cheap in multi-parallel/bridged mode would be nice though)
my parallel amp push-pull biasing scheme shown earlier in this thread should work with power chip amps as well as op amps
there is also Doug Self's "Class XD" which could be applied to power chip amps
on the smaller scale, some early generation A/DSL driver op amps have bias current setting/boost pins, presumably most of the extra bias is going to the output stage, I believe a few 10s of mA Class A output is possible with some of these chips
they were designed for low distortion (some down to -100 dB) at 100 KHz - low MHz into 25-100 OHm loads
newer ADSL chips have apparently been designed for much lower quiescent current, according to Scott Wurcer the industry has come to accept ~80 dB distortion numbers as adequate so their outputs are low bias again
there is also Doug Self's "Class XD" which could be applied to power chip amps
on the smaller scale, some early generation A/DSL driver op amps have bias current setting/boost pins, presumably most of the extra bias is going to the output stage, I believe a few 10s of mA Class A output is possible with some of these chips
they were designed for low distortion (some down to -100 dB) at 100 KHz - low MHz into 25-100 OHm loads
newer ADSL chips have apparently been designed for much lower quiescent current, according to Scott Wurcer the industry has come to accept ~80 dB distortion numbers as adequate so their outputs are low bias again
Last edited:
Member
Joined 2009
Paid Member
the DC current loading would have to come before any/all DC blocking capacitors and before the output impedance setting resistor.
I doubt the "output" is directly connected to the opamp output Pin.
Interesting part, thanks for bringing it to our attention. Looks like the output load should always be kept >2k at 5V supply i.e. the classA bias is just over 1mA. Also minimum stable gain of 30dB is something to look out for - won't be applicable to too many opamp circuits but excellent for its intended application as mic pre.
:- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.