Good afternoon all,
first post here. I'm coming to this because I have an old HH Scott 340B receiver and I've been trying to get it in top shape. I thought I had an issue with it, but it turns out the real "problem" is an impedance mismatch between its tape out jacks and basically anything I'd want to hook to it. The Scott wants anything connected to have a minimum impedance of 100K ohms, whereas my ADC Sound Shaper SS-412x EQ is 47K ohms. That is enough that just having the EQ connected and powered up, even if the EQ is bypassed, destroys the bass response of the Scott on phono input. I have the same issue with my cassette deck connected directly to the Scott. With nothing hooked to Tape Out, it sounds great, although then I obviously have no line out and can't use the 412x as a spectrum display or to provide two tape loops.
more discussion of the issue here:
Scott 340B weak bass on phono? | Audiokarma Home Audio Stereo Discussion Forums
Posting in this thread as someone suggested a headphone amp might work as a buffer, and I immediately thought of the O2. I sent an inquiry to JDS Labs and got a response that the input impedance was ">10K" when I gave more detail I was told that changing the resistors would work.
"you can see that the input stage is coupled to ground via 10k resistor. Input impedance of the NJM2068D opamp is higher, but the 10k resistor will be a limiting factor. It is safe to assume 10k. You should be able to swap these resistors to 499k if you need a higher input impedance. I have not tested this setup exactly, but would from other experiments we have conducted, there should be little to no impact on overall performance."
I'm assuming he's talking about R14 and R20 and does this seem like a logical way to approach this? I've always kind of wanted to try an O2 but never had a real need for it, if I could use it both for its intended purpose and as a buffer for my tube system, that would be fantastic.
Or is there a much simpler/cheaper buffer that would work for the Scott, and can you suggest one? Thanks much. If using something else would be a better approach, let me know and I'll start a different thread.
first post here. I'm coming to this because I have an old HH Scott 340B receiver and I've been trying to get it in top shape. I thought I had an issue with it, but it turns out the real "problem" is an impedance mismatch between its tape out jacks and basically anything I'd want to hook to it. The Scott wants anything connected to have a minimum impedance of 100K ohms, whereas my ADC Sound Shaper SS-412x EQ is 47K ohms. That is enough that just having the EQ connected and powered up, even if the EQ is bypassed, destroys the bass response of the Scott on phono input. I have the same issue with my cassette deck connected directly to the Scott. With nothing hooked to Tape Out, it sounds great, although then I obviously have no line out and can't use the 412x as a spectrum display or to provide two tape loops.
more discussion of the issue here:
Scott 340B weak bass on phono? | Audiokarma Home Audio Stereo Discussion Forums
Posting in this thread as someone suggested a headphone amp might work as a buffer, and I immediately thought of the O2. I sent an inquiry to JDS Labs and got a response that the input impedance was ">10K" when I gave more detail I was told that changing the resistors would work.
"you can see that the input stage is coupled to ground via 10k resistor. Input impedance of the NJM2068D opamp is higher, but the 10k resistor will be a limiting factor. It is safe to assume 10k. You should be able to swap these resistors to 499k if you need a higher input impedance. I have not tested this setup exactly, but would from other experiments we have conducted, there should be little to no impact on overall performance."
I'm assuming he's talking about R14 and R20 and does this seem like a logical way to approach this? I've always kind of wanted to try an O2 but never had a real need for it, if I could use it both for its intended purpose and as a buffer for my tube system, that would be fantastic.
Or is there a much simpler/cheaper buffer that would work for the Scott, and can you suggest one? Thanks much. If using something else would be a better approach, let me know and I'll start a different thread.
Welcome to diyAudio 🙂
Right... if all you need is a high impedance buffer then its probably fair to say the 02 is more complex than needed. Essentially all you need is a single opamp stage, like the 02's but designed for high impedance... which is dead easy.
The 02's stage could easily be changed to high input impedance as well.
Right... if all you need is a high impedance buffer then its probably fair to say the 02 is more complex than needed. Essentially all you need is a single opamp stage, like the 02's but designed for high impedance... which is dead easy.
The 02's stage could easily be changed to high input impedance as well.
You can increase R14 to 1 meg if you wish but I would recommend that the opamp is changed to a FET type such as an OPA2134. This is because the high input impedance and direct coupling of the gain setting components will cause a high DC offset with a standard type opamp (bjt) such as the 2068.
The 02 circuit also assumes that the output you feed it from is AC coupled.
One very practical point with any valve equipment (that is very design dependent) is to check that no high voltage transients can occur on the valve equipment outputs or inputs. This can cause issues if 'hot swapping' leads as coupling caps may have a high voltage present and that can cause damage by generating a high 'spike' as leads are connected/disconnected.
That is something that is easy to guard against with a couple of protection diodes clamping the opamp input/outputs to the opamp supply lines.
The risk is low but not unheard of.
The 02 circuit also assumes that the output you feed it from is AC coupled.
One very practical point with any valve equipment (that is very design dependent) is to check that no high voltage transients can occur on the valve equipment outputs or inputs. This can cause issues if 'hot swapping' leads as coupling caps may have a high voltage present and that can cause damage by generating a high 'spike' as leads are connected/disconnected.
That is something that is easy to guard against with a couple of protection diodes clamping the opamp input/outputs to the opamp supply lines.
The risk is low but not unheard of.
So do you think the idea of having an O2 pulling double duty as a buffer for my home system's tape out and also a portable headamp if I want it is still practical? Or will increasing the value of those resistors compromise its use for the latter somehow?
Or would you just recommend scrapping that idea and making a buffer circuit dedicated to the Scott instead? I don't have a problem with the latter, I was just hoping to find an off the shelf black box that would work, or something that I could easily modify to do what I want it to. Not having a tube tape machine (nor do I know what I would do with one if I did) the tape loop on the Scott is pretty pointless without a buffer on the output. If this is the preferable approach, can you point me toward either something that I could purchase, or a detailed schematic of what would be a good high quality solution? I do realize that I'm using a receiver almost old enough to be my dad, but it's actually pretty decent sound quality and don't want to compromise it with a hack.
Or would you just recommend scrapping that idea and making a buffer circuit dedicated to the Scott instead? I don't have a problem with the latter, I was just hoping to find an off the shelf black box that would work, or something that I could easily modify to do what I want it to. Not having a tube tape machine (nor do I know what I would do with one if I did) the tape loop on the Scott is pretty pointless without a buffer on the output. If this is the preferable approach, can you point me toward either something that I could purchase, or a detailed schematic of what would be a good high quality solution? I do realize that I'm using a receiver almost old enough to be my dad, but it's actually pretty decent sound quality and don't want to compromise it with a hack.
If you have use for it as a headphone amp then it is fine to use it as you intend 🙂
One option is for you to add another pair of sockets (RCA etc) to the 02 and to take the feed from the output of the 2068 opamp. That would give you both a line out feed (not affected by the volume control position) and also retain the normal headphone amp output.
A buffer circuit is simple. One dual opamp like a 2068 together with a couple of resistors and caps.
I can easily draw you one out of you are interested. One thing you would have to decide is how you would power it. Battery or mains? Battery is an option for such a low power circuit.
One option is for you to add another pair of sockets (RCA etc) to the 02 and to take the feed from the output of the 2068 opamp. That would give you both a line out feed (not affected by the volume control position) and also retain the normal headphone amp output.
A buffer circuit is simple. One dual opamp like a 2068 together with a couple of resistors and caps.
I can easily draw you one out of you are interested. One thing you would have to decide is how you would power it. Battery or mains? Battery is an option for such a low power circuit.
Another thought... if you have a specific tape deck in mind then perhaps it would be relatively easy to modify the input of that to a high impedance.
I think I would rather have an external buffer so that I wouldn't be limited to one unit. The way it's hooked up now, I have the tape deck hooked up to one tape loop of the EQ and a HRT DAC hooked up to the other, for listening from a laptop. Oppo BDP-95 is hooked up to "Extra" in case I want to spin a physical disc. I also have a R2R but it's hooked up to a different system.
Anyway, I'd like to hear your thoughts on what I should do but it sounds like it might be better to move this to another thread.
I did throw in a bid on an O2 on a certain auction site, so I may end up with one... what the heck, I wanted to play with it anyway.
Anyway, I'd like to hear your thoughts on what I should do but it sounds like it might be better to move this to another thread.
I did throw in a bid on an O2 on a certain auction site, so I may end up with one... what the heck, I wanted to play with it anyway.
I was thinking AC power, as my receiver has a switched power receptacle. Not really married to that idea though.
I also found searching quite a few "RTS Systems Dual Buffer Amplifier Model 444" on that auction site used and reasonably priced, but can find literally no info on it to see if it will do what I want to do, and my request to the mfgr. for a data sheet went unanswered :/ That would be cool if it'd work though as I have several half rack unit source switches so that would work well together to expand this system and look sort of intentional.
I also found searching quite a few "RTS Systems Dual Buffer Amplifier Model 444" on that auction site used and reasonably priced, but can find literally no info on it to see if it will do what I want to do, and my request to the mfgr. for a data sheet went unanswered :/ That would be cool if it'd work though as I have several half rack unit source switches so that would work well together to expand this system and look sort of intentional.
A lot depends on whether you are able to put something together yourself. The actual 'buffer' needed is super simple. This is taken from the web but it is essentially complete.
There is one opamp per channel (so a dual opamp could be used for stereo). The power supply here is a single rail meaning ground and a positive voltage. To make that work needs a couple of resistors not shown... or if powered from dual rail (like the02) then the resistor R2 would connect to ground.
And that's it. A buffer with high input impedance and low output impedance. Voltage gain of '1'.
If you want ready built then it is a case of seeing what is available.
There is one opamp per channel (so a dual opamp could be used for stereo). The power supply here is a single rail meaning ground and a positive voltage. To make that work needs a couple of resistors not shown... or if powered from dual rail (like the02) then the resistor R2 would connect to ground.
And that's it. A buffer with high input impedance and low output impedance. Voltage gain of '1'.
If you want ready built then it is a case of seeing what is available.
