| Tolu |
@KSTR
OK, then it will remain.
I am very content with the sound.
Thomas |
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| arupg |
Here's a SALUTE to CarlosFM for his thumb-rule formula of approx. 8.0 - 8.5mA biasing.
I am using the cousin of 627, the OPA2107(dual) and
1st I biased it lightly at 5.5mA - it started to sing.....
Next, I pushed this to 6.0mA - it was definitely singing.....
Then again, I said last try at 7.0mA - the singing became so much better
And Finally I told myself, what the heck, push it to 8.0mA - it has now seduced me.....
I did a lot of R&D but CarlosFM did much more or else he confidently wont keep saying to bias at 8.0 - 8.5mA.
Hats off to you Carlos!! I love this romance!
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Arup |
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| KP11520 |
Hi arupg,
That is nice to hear (for you anyway!).
Can you explain to me how you measure the bias?
Thanks and I hope to be making the same claims!
Regards//Keith |
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| Ryssen |
| quote: | | push it to 8.0mA - it has now seduced me..... |
Hot?
I once used 10mA...:hot: |
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| arupg |
Hi Keith:
I checked the supply on the pins, i.e. 4 & 8 as its a dual opamp in my case. Divide the number & I got 12.14V. So, its a +/- 12V PS.
Using V = I x R; for 5.5mA bias, I used (12/0.0055=2181) a 2.2k resistor. For 6mA, it was 2k. For 7mA I used 1.8k and finally for 8mA it was 1.5k.
Arup |
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| arupg |
Ryssen:
On the OPA2107??? The 627 can take it but NOT the 2107..... |
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| Ryssen |
It was a 627..;)
But I am using 7mA on LM4562 and AD827.Is working.:) Havent tried higher. |
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| KP11520 |
So it was by using the formula and not something you measured!
I guessed the formula helped..... Thank matjans!
I was thinkiing you could measure it with a DMM or something!
My CDP has +/- 15 volts and I figured 1.8K resistor between the V- and output would do it then I was intrigued by the JFET Cascode. I have already made them and need some time to install them and other mods while the PCB is out.
I am still deciding which output caps to replace the stock ones.
Thanks!
Regards//Keith |
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| analog_sa |
| quote: | Originally posted by KP11520
So it was by using the formula and not something you measured!
I guessed the formula helped..... Thank matjans!
I was thinkiing you could measure it with a DMM or something!
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It may be beneficial to research Ohm's law before cascodes. It may even become obvious how to measure the current "with a DMM or something" |
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| KP11520 |
Hey analog_sa
In mid April, I decided to start learning about DIY AND electronics. I think I have moved along OK in four months. I guess the statement about the DMM or something really got to you. So it was stupid (I do that every now and then). Now I'll bet if I search every thread you ever posted, I'll bet you have done that too (with probably more than four months experience when you posted to the thread, and I have to laugh when I say experience on my part, it's more like I was groping).
Yes you can measure it with a DMM and not something. Although knowing Ohm's law helps, it doesn't help with the actual placing of the probes in a series with an op amp in the pcb. That I am a little reluctant about doing, in my groping I already fried one OPA627 somewhere close to month number 3 in my four months of groping along!
I know you said you don't like Class A Biased op amps in the output stage. I see so much conflicting stuff about it, I am choosing to learn it myself. Don't be insulted that I am still looking for answers. I really do want to understand this completely! Everytime I learn something at least some of it applies to the next thing I have to learn. This is the best way and the more I learn, the less stupid things I will write!
Regards//Keith |
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| analog_sa |
Keith
I have no problem with the class A biasing. As any other contentious topic in audio the best approach is to build and listen.
So, what's taking you so long? Building cascodes, conducting opinion surveys... For something which takes 5 minutes.
How to measure the current? Connect an ammeter in series with the current setting resistor (or CCS) just to make sure the correct current is running through the output and sit down for a listen.
The best place to learn basic electronics is not a diy forum. You really need a few textbooks and sufficient motivation to read them. |
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| Ryssen |
| quote: | | How to measure the current? Connect an ammeter in series with the current setting resistor (or CCS) |
Or messaure the voltage over the resistor,devide it with the resistor.. |
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| IanAS |
"Or measure the voltage over the resistor, devide it with the resistor" value :)
12 volts on one side and 0 volts on the other side = 12 Volts dropped across the resistor.
1500 Ohms
I = V ÷ R
That means it must be 0.008 Amps. |
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| AndrewT |
| quote: | Originally posted by arupg
..... So, its a +/- 12V PS.
Using V = I x R; for 5.5mA bias, I used (12/0.0055=2181) a 2.2k resistor. For 6mA, it was 2k. For 7mA I used 1.8k and finally for 8mA it was 1.5k..... |
| quote: | Originally posted by KP11520
...............My CDP has +/- 15 volts and I figured 1.8K resistor between the V- and output would do it then................ |
Hi,
careful.
15V in lieu of 12V increases the dissipation by 120mW instead of just 96mW.
Those extra mW increase the temperature of the whole chip and any more may decrease performance rather than increase it. |
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| arupg |
Very true Andrew & appreciate you pointing towards it. The transition from +/- 12V to 15V to 18V PS is NOT a linear curve.
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KP11520:
15V / 0.008A = 1875ohms
So a 1.8k will actually bias it higher than 8mA. I'd say you take a shot with a 2k (1% Tol) metal-film resistor which would bias to 7.5mA. If this is working in that circuit the results would already be there and you should be able to hear/feel it. At the same time, its safe-playing.
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Arup |
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| Tolu |
Hi Ryssen
what can you say about the audible effect in biasing the LM4562? How has sound changed?
I am using the single version LME49710 in my preamp.
Regards
Thomas |
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| IanAS |
That 120mW is the dissipation from the resistor. A 'nornal' one is rated at 600mW at 70°C.
Does it mean that the chip is dissipating the same mW? A chip has a much larger surface area than the 0.6W resistor and semiconductors seem to work just fine at higher temps just their life span is shortened.
I used the finger test to feel how warm the chip was. A surface mount non-dual OpAmp (AD825) seemed OK with a 2K and ± 15V supplies. Maybe it was at 40°C. |
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| Ryssen |
| quote: | | what can you say about the audible effect in biasing the LM4562? How has sound changed? |
I use the opamp before a discrete buffer stage in my headphone amp,and I have a switch on the bias,wich use jfets.
I can´t say that I here any difference with or without bias...:cool: :whazzat:
Maybee there would be a difference if the buffert where not there..
Maybee you here diference in your preamp,if you only have the opams,not a buffer.
Try it and report..:) |
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| AndrewT |
| quote: | Originally posted by AndrewT
15V in lieu of 12V increases the dissipation by 120mW instead of just 96mW.
Those extra mW increase the temperature of the whole chip |
| quote: | Originally posted by IanAS
Does it mean that the chip is dissipating the same mW?.............. A surface mount non-dual OpAmp (AD825) seemed OK with a 2K and ± 15V supplies. Maybe it was at 40°C. |
Hi, a normal opamp runs warm when the quasi output stage is running with just a tiny bias current (my guess=200uA).
Running the chip on high voltage rails takes the opamp to much higher internal temperatures.
Now add some 100mW or so of extra internal dissipation due to converting the output stage to single ended ClassA and the chip will run hot on low voltage rails and very hot on high voltage rails.
Q.
if the resistor loaded opamp works well with 8mA. What level of bias needs to flow if the output is loaded with a CCS? Is it half? i.e.4mA into a CCS? |
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| Ryssen |
| quote: | | What level of bias needs to flow if the output is loaded with a CCS? |
Isn´t that depending on the load?
Or did you think of something else? |
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| AndrewT |
| quote: | Originally posted by Ryssen
Isn´t that depending on the load?
Or did you think of something else? |
I don't think so.
The 8mA recommendation will be load dependant, but was unspecified.
For the same unspecified conditions, I am asking whether using a CCS to pull the output stage into ClassA could operate with just half the bias current.
This should be exactly the same as any conventional single ended output stage. The least efficient are the resistor tail loading. The more efficient are the CCS tail load.
I have a feeling that if a CCS were substituted for the 1k5 to 2k0 resistor then bias and heat could be halved.
If you look back at other reports you often find CCS give superior sound quality and this may be related to the lower heating within the opamp. |
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| Ryssen |
| quote: | | The other benefit of using a JFET instead of a plain resistor is that the input impedance of a JFET is up in the megohms range, so the op-amp still sees a near-ideal load situation. |
Same as with CCS on tubes,I´ve learned.:scratch1:
| quote: | | If you put a second JFET between the op-amp's output and a JFET configured as above, you have a "cascode" configuration. (See the schematic below.) The JFET cascode has a very interesting interaction. Q1 represents a fairly stable current bias for Q2, which means that Q2 will tend to pass a fairly stable voltage level from the op-amp's output through to Q1. Conversely, Q1 now sees a more stable voltage than it would if it were connected straight to the op-amp's output, so the current flowing through it doesn't vary by as much, which helps Q2 to maintain a steady voltage level. Thus, each JFET acts to help maintain stability in the other JFET, leading to a nearly constant current level through the cascode despite input voltage variation. In my tests, current variation for a properly-configured cascode was roughly in the +/-0.5% range. That's just a few tens of microamps of current difference for 2N5484s! |
More stable current with jfets,doesn´t a CCS isolate the negative from the output better?
| quote: | | A notable feature of the cascode configuration is that it will have a lower current draw than that of the Q1 alone. This can be an advantage or an annoyance, depending on your application. |
Lower current draw,hmm,that must depend how you set it up,(resistor?)?
This is where I coppied text from:
http://tangentsoft.net/audio/opamp-bias.html
Schema |
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| KP11520 |
Hyperlink to the 2N5484 JFETs page down to bottom: http://tangentsoft.net/shop/
This is how I built it (JFET Cascode) for my situation on the attachment:
I am trying to learn (and please don't tell me) what impact changing the JFETs in similar matched pairs has on the mA. Maybe it doesn't matter, I will learn that on my own as well.
Hopefully this can help someone! |
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