anatech said:
Hi,
It has an open loop gain of 140dB, so it must need some negative feedback? In the datasheet they don't say how to calculate feedback resistor values for desired gain. How would I work out what feedback resistor value I would want for a gain of 1?
I've been wondering if it would be practical to use a variable resistor in the negative feedback to control the volume. Would this be likely to affect distortion much over a sensible range of output levels? i.e. no output to about 2V.
Sorry for the noob questions but I have not done any active designs before.
Well.... I'm a big fan of transformer coupled circuits, no matter what the graphs and opamps say. 
That's always been the case in pro audio and Hi-Fi for my ears. Put a transformer in and it sounds better to me. "ahhh... that's better...."
Maybe it "shouldn't" be that way - but to my ears, it is.
That's always been the case in pro audio and Hi-Fi for my ears. Put a transformer in and it sounds better to me. "ahhh... that's better...."
Maybe it "shouldn't" be that way - but to my ears, it is.
Hi Tenson,
Here is the data sheet for the BUF634 from the TI web site. I am pretty sure I should read that data sheet, but haven't had the time to refresh my memory.
Data sheets are great in that they tell you what the device is expected to do, and how to achieve that performance. Application notes will extend that knowledge and may pay more attention to "gotchas", or what we like to call "undocumented features". Basically how to avoid problems.
Hi panomaniac,
-Chris
Here is the data sheet for the BUF634 from the TI web site. I am pretty sure I should read that data sheet, but haven't had the time to refresh my memory.
Data sheets are great in that they tell you what the device is expected to do, and how to achieve that performance. Application notes will extend that knowledge and may pay more attention to "gotchas", or what we like to call "undocumented features". Basically how to avoid problems.
Hi panomaniac,
I've heard transformers sound both good and bad. Same with electronics. It depends on many things as to which is the better choice. Sending signals long distances to other properly designed equipment would have me reaching for transformers, or at least thinking of using them. I've used them in the past and they performed well. I've also removed transformers to correct problems.
Too many variables with ears. They are a good guide if you also look at measurements. Execution of circuits is often suspect in some professional gear. Especially with effects units or "semi pro" equipment. My favorite bad interface is the -10 and +4 output assembly. Normally using a combined 1/4" and XLR jack. Avoid.
-Chris
What is semi-pro? A gear from Guitar Center usually goes to professionals, when semi-pro folks like doctors, lawyers, drug de... sorry, pharmaceutical representatives buy a gear from my partner who sells high-end stuff only, or contact me directly to order something special...
Is Behringer's stuff pro, or semi-pro? It is cheap... But sometimes it is very nice. I can't name those who build a junk for professionals, I don't want their angry calls anymore... But "pro" is really a can of worms, if to know where to look and what to check...
Is Behringer's stuff pro, or semi-pro? It is cheap... But sometimes it is very nice. I can't name those who build a junk for professionals, I don't want their angry calls anymore... But "pro" is really a can of worms, if to know where to look and what to check...
Bit off-topic eh boys? Did we solve this problem yet?
The buffer may make a difference but it may not...if you THINK it will, it probably will!
I suggest you just wire in the volume control, then start listening to music. Then you can think about whether you want to add a buffer. Then listen to some more music. If you do add the buffer, you can use the same volume control.anatech said:
The buffer may make a difference but it may not...if you THINK it will, it probably will!
Hi,
I can listen to music at the moment so I'm in no big rush.
I am still thinking of going down the op-amp route. Let me run what I'm thinking passed you guys and ask for a little advice.
This is basically my plan.
The first voltage dividers R1 and R2 I put there in hope of loading the DAC with a lower impedance. It has been said that the AKM DAC likes a load of around 1K, so the high input impedance of the op-amp may not suit directly connected.
I need help working out what value resistors I need to use to load the DAC with 1K and to give as little signal attenuation as possible?
Then I have caps to block DC. and the op-amp with a gain of 1 (see I read up on negative feedback!).
Then the next part I plan to use a shunt attenuator (because I can make one with a 6 layer 24 position switch I have) after the op-amp for level control.
The op-amp can drive low loads so am I right thinking I should go for a low series resistor like 5K so that the source feeding the amp is not too high? Or should I just use a standard 10K shunt setup? Alternatively I could make my switch in to a simple adjustable series resistor, would that be better?
Advice appreciated
I can listen to music at the moment so I'm in no big rush.
I am still thinking of going down the op-amp route. Let me run what I'm thinking passed you guys and ask for a little advice.
This is basically my plan.
An externally hosted image should be here but it was not working when we last tested it.
The first voltage dividers R1 and R2 I put there in hope of loading the DAC with a lower impedance. It has been said that the AKM DAC likes a load of around 1K, so the high input impedance of the op-amp may not suit directly connected.
I need help working out what value resistors I need to use to load the DAC with 1K and to give as little signal attenuation as possible?
Then I have caps to block DC. and the op-amp with a gain of 1 (see I read up on negative feedback!).
Then the next part I plan to use a shunt attenuator (because I can make one with a 6 layer 24 position switch I have) after the op-amp for level control.
The op-amp can drive low loads so am I right thinking I should go for a low series resistor like 5K so that the source feeding the amp is not too high? Or should I just use a standard 10K shunt setup? Alternatively I could make my switch in to a simple adjustable series resistor, would that be better?
Advice appreciated
Hi Anatoliy,
Semi-pro equipment is normally a great deal less expensive that the "real" gear. The existence of RCA connectors normally confirms this. If you have an output that is supposed to be -10 dbV and +4 dBm at the same time, you are looking at semi-pro. BTW, that output is not even close to being balanced.
A good example would be a Studer tape machine compared to a Fostex machine. Tascam can straddle that definition, better that Fostex (by miles!), but inferior to Studer (not even close).
Hi paulb,
now.
-Chris
Semi-pro equipment is normally a great deal less expensive that the "real" gear. The existence of RCA connectors normally confirms this. If you have an output that is supposed to be -10 dbV and +4 dBm at the same time, you are looking at semi-pro. BTW, that output is not even close to being balanced.
A good example would be a Studer tape machine compared to a Fostex machine. Tascam can straddle that definition, better that Fostex (by miles!), but inferior to Studer (not even close).
Surely sir!, you jest!
Hi paulb,
I've been promoting an open mind about this. Just consider a buffer. A buffer will make a difference as soon as the input impedance of the amplifier is lowered. More insidious is the distortion that can be created if your input stage has a non-linea impedance. Nakamichi came out with "HTA" I think it was, but Marantz had that figured out in 1968 when they designed the model 500 amplifier. That was to insert a linear buffer ahead of the BJT differential pair. The trick is to drive a BJT input pair with a low impedance. A light should be coming on right about
now.-Chris
Tenson said:
Depending on where do you add it. If you add it before the pot you obviously will loose half of the power; if you add it after you may attenuate it depending on a load resistance. For modern gear you should not worry about such possible attenuation since even modern pro gear has input resistance much more than 600 Ohm used long time ago. Speaking of driving of long cables, you may assume 50 pF per meter. 500 pF per 10 meters mean 1 kiloohm on some 300 kiloHertz frequency. However, if you design a high-end gear for pro usage, I would suggest you to use a line transformer, but the majority of modern pro gear use opamps, so even in high-end bass guitar amps I use non-symmetrical impedance balanced line outs that is more than plenty for studio recording purposes.
...I don't really have a clue what you just said!
The L-pad will go after the DAC chip, and before the op-amp input so that the DAC sees a lower load.
I can afford some attenuation because I can use a gain higher than 1 in the op-amp, but I see no reason to attenuate more than I need to. The important thing is to load the DAC nicely.
Oh one other thing. I read on Doug Self's website that the LM4562 has lower distortion using shunt feedback. If I simply connect the output to the inverting input for a gain of 1, is this going to give the same common-mode disto problems as series feedback?
http://www.dself.dsl.pipex.com/ampins/webbop/LM4562.htm
The L-pad will go after the DAC chip, and before the op-amp input so that the DAC sees a lower load.
I can afford some attenuation because I can use a gain higher than 1 in the op-amp, but I see no reason to attenuate more than I need to. The important thing is to load the DAC nicely.
Oh one other thing. I read on Doug Self's website that the LM4562 has lower distortion using shunt feedback. If I simply connect the output to the inverting input for a gain of 1, is this going to give the same common-mode disto problems as series feedback?
http://www.dself.dsl.pipex.com/ampins/webbop/LM4562.htm
Hi paulb,
Current output DACs have a happy low impedance they like to see, this becomes the I/V converter. What that value is for this one, I haven't a clue. If this is a voltage output DAC, I would be surprised if it didn't want to see 10K or higher.
I guess someone could always look up the data sheet, or link to it.
-Chris
Current output DACs have a happy low impedance they like to see, this becomes the I/V converter. What that value is for this one, I haven't a clue. If this is a voltage output DAC, I would be surprised if it didn't want to see 10K or higher.
I guess someone could always look up the data sheet, or link to it.
-Chris
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