The current gain is extremely high. For example at 1 volt rms output you might realistically expect the output to support a load impedance of just a few ohms. So the output could deliver many tens of millamps. The input current into the input transistors will be a few tens of microamps.
So the current gain is high, I'm guessing in the order of 70db or more. Remember the current 'gain' in this sense depends entirely on the load.
So the current gain is high, I'm guessing in the order of 70db or more. Remember the current 'gain' in this sense depends entirely on the load.
I really should read the entire OP first. I was calculating voltage gain; came up with 6.3× without looking up all the particular transistor fine-details. Then I read 5.8× in the OP's comment.
Well, at least “in the ballpark”! Makes sense: the note in the box calls for 200 mV test conditions. 5.8× to 6.3× would deliver a nice 1200 mV or so nominal. Its a good signal voltage.
Mooly, as he almost-always is, is right. 70+ dB (103.5 ≈ 3000) is about right. But that's also kind of what one expects from a sub-board called 'volume control'
GoatGuy
Well, at least “in the ballpark”! Makes sense: the note in the box calls for 200 mV test conditions. 5.8× to 6.3× would deliver a nice 1200 mV or so nominal. Its a good signal voltage.
Mooly, as he almost-always is, is right. 70+ dB (103.5 ≈ 3000) is about right. But that's also kind of what one expects from a sub-board called 'volume control'
GoatGuy
Why do you want to know?
How much do you think you need?
A 4 transistor cascade, with assumptions from mild to wild, has current gain of 10,000 to many million. This is probably not too significant.
The circuit as presented, the output just goes to a dot "TO_L". There is NO load, no output current. Current gain is zero (not unity).
The "maximum" current output would be with dot "TO_L" shorted to ground. R5039 220r limits the current. Input is let's say R5003 56K. If voltage gain were unity, then the 220r would get 56K/220 or 255X the input current. But the voltage gain is say 6X, and 255*6 is 1,500 current gain.
It is not certain the amp can be happy driving R5039 220r into a short. But that would be silly anyway.
Let us assume the "TO_L" point is a power amp and they had heaps of 56K resistors, so the load is 56K. Then Rin=Rout and with voltage gain of 6 the current gain is just 6.
Actually this stage is self-loaded by R5041 R5043 R5045 or 590 Ohms, which is 95X more load than the 56K input. Taking voltage gain of 6, current gain is 569. (We could also observe that R5041 follows the input, 56K:100 is 560 current gain.)
Since the current gain needed is under 600, and the transistors have current gain over 10,000 on tap, we don't need an exact number for internal current gain.
ah.... actually, the naked amp has two current gains. From C5001 on there are four stages cascoded. But the NFB is applied one stage along, and at a point where stage 2 gives unity current gain. Current gain to 2nd stage may only be 2.1. We do have the final stage working at full hFE which for KSC1845F is 300-600. So current gain around the NFB loop may "only" be 600. Quite close to the 560 needed to drive the NFB loop. Inspired design? Oversight? My mis-understanding?
IAC, I think the current gain is ample for what it needs to do.
There is a voltage
What do you want with the current gain???
This is a preamplifier stage, which was developed for voltage gain. Current gain depended by the load. Change the load from 1k to 1Meg will change the current gain with 1000...
Sajti
Thank all for your reply.
If I understand from the above, by changing R5041 and R5042 from 100 ohms to 470 ohms, then this preamp stage would reduce its voltage gain from 15.4db to 6db while keeping the original current gain if there is no change in the load /components in the subsequent main amplifier stage (I believe this would be 47K ohms).
I am trying to achieve a voltage gain reduction, while maintaining the existing current gain of the entire amplifier. Of course, this applies for the same volume position of the amplifier.
I am a novice in electronics / DIY, please forgive me as I am trying to learn at the same time.
A side question-if I increase C5005 and C5006 from 10uf to lets say, 47uf, what does this achieve?
Thanks in advance.
If I understand from the above, by changing R5041 and R5042 from 100 ohms to 470 ohms, then this preamp stage would reduce its voltage gain from 15.4db to 6db while keeping the original current gain if there is no change in the load /components in the subsequent main amplifier stage (I believe this would be 47K ohms).
I am trying to achieve a voltage gain reduction, while maintaining the existing current gain of the entire amplifier. Of course, this applies for the same volume position of the amplifier.
I am a novice in electronics / DIY, please forgive me as I am trying to learn at the same time.
A side question-if I increase C5005 and C5006 from 10uf to lets say, 47uf, what does this achieve?
Thanks in advance.
Changing the voltage gain of 'complex' circuits using feedback isn't something to undertake lightly because the change of gain can effect stability margins and the way the amplifier handles signals of fast rise times.
My first though was to say this circuit would be OK but I also see there is a network consisting of R5029, R5031 and C5007 that will be related to the above. 1meg ohm seems implausibly high at first glance, but if those are indeed the values used then you need to question how changing the gain would affect the original designs performance.
C5005/6 are a standard value ideally suited to their function here which is just to maintain a low impedance between the base of the output transistors with rising frequency, and so help with maintaining open loop gain and low distortion as frequency rises. No benefit what so ever in altering these.
My first though was to say this circuit would be OK but I also see there is a network consisting of R5029, R5031 and C5007 that will be related to the above. 1meg ohm seems implausibly high at first glance, but if those are indeed the values used then you need to question how changing the gain would affect the original designs performance.
C5005/6 are a standard value ideally suited to their function here which is just to maintain a low impedance between the base of the output transistors with rising frequency, and so help with maintaining open loop gain and low distortion as frequency rises. No benefit what so ever in altering these.
This is CFA amplifier. If You increase the 100ohms to 470, You will decrease the open loop gain as well, so there will be no problem with the stability.
I use something like that in my V4 amplifier.
Sajti
Hi, everyone!
I have obtained the resistors (to change from original 100 ohm to 470 ohm) and have installed them. Double checked the installation and seems to be fine.
Result - reduced dynamics and bass (quite a lot!), front-to-back soundstage totally collapsed and turned to 2D soundstage. Vocals that are supposed to be focused in the middle are hollow.
Is there anything that could be done?
Thanks!
I have obtained the resistors (to change from original 100 ohm to 470 ohm) and have installed them. Double checked the installation and seems to be fine.
Result - reduced dynamics and bass (quite a lot!), front-to-back soundstage totally collapsed and turned to 2D soundstage. Vocals that are supposed to be focused in the middle are hollow.
Is there anything that could be done?
Thanks!
**Update**
-I have put back 100 ohms back to R5041 / R5042, returning to stock.
-Then I have changed R5037 / R5038 from 100K to 180K (got a Bourns 3296 trimmer). Much improved bass extension than stock!
-As increasing the volume past 9 o'clock increase the treble glare even in stock form, I experiment removing C5007 / C5008 / C5003 / C5004 which seems to tame it down just a bit, but not enough - any ideas?
Thanks to everyone who chimed in, much appreciated!
Philip C.
-I have put back 100 ohms back to R5041 / R5042, returning to stock.
-Then I have changed R5037 / R5038 from 100K to 180K (got a Bourns 3296 trimmer). Much improved bass extension than stock!
-As increasing the volume past 9 o'clock increase the treble glare even in stock form, I experiment removing C5007 / C5008 / C5003 / C5004 which seems to tame it down just a bit, but not enough - any ideas?
Thanks to everyone who chimed in, much appreciated!
Philip C.
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