Well, I've got some more data. My SMSL DAC arrived today with has a 100ohm RCA output and 200ohm XLR. I ran the same tests and got the exact same results as with the CD Player. The pre-amp handled the 100ohm load just fine with normal frequency response but couldn't even handle the 200ohm load from the balanced output of the DAC. One thing I find confusing is that the 530ohm output from the CD Player caused the nearly the same high frequency drop out as the 200ohm balanced load. Perhaps the input impedance is different for each.
@rayma I wanted to do the testing and give the manufacturer some additional time to respond to my request for warranty support before outing the company but he continues to ignore me. I'll pull the pre-amp out and get some pictures of the circuit later today.
Also the charts look different than the ones yesterday as I measured those at my listening position and today's are measured near field at the speaker to eliminate room interactions and keep them more consistent. You can see that all the traces are matched within 0.4 dB where they aren't affected by the EQ or the high frequency drop out.
@rayma I wanted to do the testing and give the manufacturer some additional time to respond to my request for warranty support before outing the company but he continues to ignore me. I'll pull the pre-amp out and get some pictures of the circuit later today.
Also the charts look different than the ones yesterday as I measured those at my listening position and today's are measured near field at the speaker to eliminate room interactions and keep them more consistent. You can see that all the traces are matched within 0.4 dB where they aren't affected by the EQ or the high frequency drop out.
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@rayma
Here's a link to a google drive folder with high res pictures of the pre-amp internals. The balanced power supply is in a completely separate 40lbs chassis and attaches to the pre-amp via the umbilical connection at the rear. It has 200,000 micro farad of poly caps and separate transformers for the high/low voltage filaments.
Initially with the TCV by themselves there was a huge problem with microphonics as the original design did not properly consider the output power of the 300B when used as a pre-amp tube. I could clap my hands 5 ft away and hear it through the speakers. I sent it back and while refusing to tell me what exactly was wrong their solution set the TCV at max and then installed the 50K ALPS volume controls on the output to control gain. I was refused the option of a refund or a new unit when they reached a final design.
To be fair it does sound absolutely amazing when functioning properly.
The only resistors are the visible cemented ones near the input/output and there's a resistor under the 6 central caps 20 ohm on the small and 26 Ohm on the 4 larger.
https://drive.google.com/drive/folders/126LqgPa2V0uoV2sMVu2Jv17w-KisMz-n?usp=drive_link
Here's a link to a google drive folder with high res pictures of the pre-amp internals. The balanced power supply is in a completely separate 40lbs chassis and attaches to the pre-amp via the umbilical connection at the rear. It has 200,000 micro farad of poly caps and separate transformers for the high/low voltage filaments.
Initially with the TCV by themselves there was a huge problem with microphonics as the original design did not properly consider the output power of the 300B when used as a pre-amp tube. I could clap my hands 5 ft away and hear it through the speakers. I sent it back and while refusing to tell me what exactly was wrong their solution set the TCV at max and then installed the 50K ALPS volume controls on the output to control gain. I was refused the option of a refund or a new unit when they reached a final design.
To be fair it does sound absolutely amazing when functioning properly.
The only resistors are the visible cemented ones near the input/output and there's a resistor under the 6 central caps 20 ohm on the small and 26 Ohm on the 4 larger.
https://drive.google.com/drive/folders/126LqgPa2V0uoV2sMVu2Jv17w-KisMz-n?usp=drive_link
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This is a PREAMP, right? What power amplifier do you use with it?
Anyway, it seems to me that they should remove the TVC entirely, rather than just turning it up to the maximum output.
At the maximum output position, it is only degrading the input, not changing the level or doing anything useful.
This appears to be an expensive preamp, and there is no reason to retain the TVC if it is permanently set to maximum.
Is the company in Canada, or do you have to ship this for service? With the power supply?
Anyway, it seems to me that they should remove the TVC entirely, rather than just turning it up to the maximum output.
At the maximum output position, it is only degrading the input, not changing the level or doing anything useful.
This appears to be an expensive preamp, and there is no reason to retain the TVC if it is permanently set to maximum.
Is the company in Canada, or do you have to ship this for service? With the power supply?
Yeah, it's a pre-amp. I run it with 300B set monoblock amps from the same company. Yes, it's an extremely expensive unit and you're bringing up the same points I brought up with the owner of the company.
I'm the one who discovered the inherent flaw in the first design so when I sent it back for them to "fix" it their solution was as you see it. I specifically complained about the fix but was ignored and informed that as far as they were concerned they had satisfied their responsibility. I think that perhaps they are trying to make sure that the other people who bought the amp don't find out about the issues so that they don't have to deal with a recall.
I assume they are going to re-design it without the TCV. I asked for the re-designed amp when available and this request was also denied. I haven't made any public comments regarding the terrible customer experience up until now hoping to maintain some semblance of a civil relationship(given he's the only source of parts to repair both the very expensive pre and power amps that I own) but now that he's refusing to honor the warranty on a many thousand dollar pre-amp I have no reason to hold back anymore.
The company is in Canada and yes I have to ship two large 45lb boxes every time I want it serviced. However, right now he's completely ignoring me with regards to warranty/technical support even though I purchased it less than 6 months ago(and have only had it in my possession for 3 because of multiple problems).
Do you think the two volume controls could be the reason why it can't deal with normal output impedance?
I'm the one who discovered the inherent flaw in the first design so when I sent it back for them to "fix" it their solution was as you see it. I specifically complained about the fix but was ignored and informed that as far as they were concerned they had satisfied their responsibility. I think that perhaps they are trying to make sure that the other people who bought the amp don't find out about the issues so that they don't have to deal with a recall.
I assume they are going to re-design it without the TCV. I asked for the re-designed amp when available and this request was also denied. I haven't made any public comments regarding the terrible customer experience up until now hoping to maintain some semblance of a civil relationship(given he's the only source of parts to repair both the very expensive pre and power amps that I own) but now that he's refusing to honor the warranty on a many thousand dollar pre-amp I have no reason to hold back anymore.
The company is in Canada and yes I have to ship two large 45lb boxes every time I want it serviced. However, right now he's completely ignoring me with regards to warranty/technical support even though I purchased it less than 6 months ago(and have only had it in my possession for 3 because of multiple problems).
Do you think the two volume controls could be the reason why it can't deal with normal output impedance?
First, he is in violation of honoring the stated warranty, which is a serious legal matter that he WILL have to deal with.
Second, there is little doubt that the unusual input circuit is the problem; either the TVC, the transformer, or both.
If you can post a photo of the input transformer, showing any markings, that would be helpful for analysis of the problem.
In all candor, all the parts in these units are almost certainly readily available from other sources.
Small manufacturers can seldom afford custom made parts, because they would be extremely expensive per unit.
Second, there is little doubt that the unusual input circuit is the problem; either the TVC, the transformer, or both.
If you can post a photo of the input transformer, showing any markings, that would be helpful for analysis of the problem.
In all candor, all the parts in these units are almost certainly readily available from other sources.
Small manufacturers can seldom afford custom made parts, because they would be extremely expensive per unit.
Yeah, I thought about that initially but after four months of just trying to get my system up and running again(and thinking at the time that it was functioning normally if using a compromised design) I just wanted to listen to music again and not think about it anymore. Now I have to open the box of worms again because it is clearly not functioning properly and I can't even get him to respond. It's also false advertising since I purchased a pre-amp that was supposed to have a very nice TCV and not a $30 ALPS blue. At this point it looks like I have no other option but to take direct action against the company. I was hoping it would be something I could fix relatively easily myself with guidance from the manufacturer or online resources.
I'd like to be able to actually measure the input impedance as well as that is advertised as 100,000ohms. If it is, in actuality, extremely low then that could explain the high frequency drop out with the higher output impedance as well could it not?
Unfortunately the transformers are all custom wound units made specifically for this pre-amp and I don't believe they have any identifying marks on them. I'll check though. I owned the previous version of the pre-amp which used a different tube but a similar TCV on the input(but obviously not another pot on the output) and I don't believe it had this issue.
I'd be more suspicious of something in their "fix" that they didn't account for causing this issue.
I'd like to be able to actually measure the input impedance as well as that is advertised as 100,000ohms. If it is, in actuality, extremely low then that could explain the high frequency drop out with the higher output impedance as well could it not?
Unfortunately the transformers are all custom wound units made specifically for this pre-amp and I don't believe they have any identifying marks on them. I'll check though. I owned the previous version of the pre-amp which used a different tube but a similar TCV on the input(but obviously not another pot on the output) and I don't believe it had this issue.
I'd be more suspicious of something in their "fix" that they didn't account for causing this issue.
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The problem is at the input, not at the output. But if you draw a schematic, it should be possible to debug the circuit.
There is no way the preamp input impedance is 100k, not with the TVC and transformer there.
There is no way the preamp input impedance is 100k, not with the TVC and transformer there.
Well, the way it seems to me(and I could easily be wrong) is that the signal loss could be taking place anywhere in the circuit between the input and output right? Perhaps I can trace the voltage and see where the level drops below the expected amount? I can measure it at the input and output with a low impedance source and then with a higher impedance source to see get the difference and then attempt to locate where that change takes place.
I don't have any experience drawing schematics or circuit diagrams but I'll see if I can find a crash course online.
Well, I just measured the input impedance on the pre-amplifier using variable resistance between the source(1khz sine wave at fixed voltage) and the input of the pre-amp while measuring voltage at the output of the pre-amp. I set it up so that the voltage at the output read exactly 0.5v and then inserted different value resistors between the source and pre-amp until the output was halved meaning that the resistor and input impedance of the amp was sharing the load equally.
First I tested with a 100k resistor 0.500v turned into 0.007v clearly the input impedance isn't even close to 100k ohm
50k resistor - 0.125v so it's not even 50k
10k resistor - 0.323v so we know it's between 10k and 50k
19.7k resistor - 0.234v ....so it's under 20k!
17k resistor - 0.251v Bingo!
So not exactly the 100k ohm input impedance advertised by the manufacturer. This jives with the fact that the Rotel pre-amp with 47k ohm input impedance was able to successfully reproduce the signal without impact but the supposed 100k ohm pre-amp wasn't.
I don't have any experience drawing schematics or circuit diagrams but I'll see if I can find a crash course online.
Well, I just measured the input impedance on the pre-amplifier using variable resistance between the source(1khz sine wave at fixed voltage) and the input of the pre-amp while measuring voltage at the output of the pre-amp. I set it up so that the voltage at the output read exactly 0.5v and then inserted different value resistors between the source and pre-amp until the output was halved meaning that the resistor and input impedance of the amp was sharing the load equally.
First I tested with a 100k resistor 0.500v turned into 0.007v clearly the input impedance isn't even close to 100k ohm
50k resistor - 0.125v so it's not even 50k
10k resistor - 0.323v so we know it's between 10k and 50k
19.7k resistor - 0.234v ....so it's under 20k!
17k resistor - 0.251v Bingo!
So not exactly the 100k ohm input impedance advertised by the manufacturer. This jives with the fact that the Rotel pre-amp with 47k ohm input impedance was able to successfully reproduce the signal without impact but the supposed 100k ohm pre-amp wasn't.
Yes, audio transformers will be much less than 100k impedance. Usually from 600 ohms to 10k ohms.
Would this alone explain why there's high frequency drop out with the higher output impedance sources but not with a 100ohm source? There are tube amps with input transformers that have higher input impedance. How is this achieved?
Not really, a device with 100 ohm output impedance should be able to drive 17k very easily.
I am not aware of any transformer input audio amplifiers with an input impedance higher than 10k.
Wide bandwidth audio transformers are difficult to build higher than 10k. For this reason, the impedance
levels are deliberately kept low as possible to extend the useful bandwidth.
I am not aware of any transformer input audio amplifiers with an input impedance higher than 10k.
Wide bandwidth audio transformers are difficult to build higher than 10k. For this reason, the impedance
levels are deliberately kept low as possible to extend the useful bandwidth.
It actually works as expected with 100ohm but with the 530ohm output impedance from the CD player it starts falling off at 2Khz and is down 4dB at 10khz.
I'm guessing the input impedance will be variable through the frequency range and fall off at the higher frequencies(I tested at 1Khz). I'll probably run the same test at 10khz to see if the input impedance goes down.
I finally received a call this afternoon from the technical support guy for the company who says this is normal and expected but he didn't know why they published the input impedance at 100,000ohm(He wasn't involved with this). He also said that future pre-amps are still going to use the same input transformers but just not tap them for the TCV and continue to use the same volume control configuration on the output so there shouldn't be any difference in sound quality.
I'm guessing the input impedance will be variable through the frequency range and fall off at the higher frequencies(I tested at 1Khz). I'll probably run the same test at 10khz to see if the input impedance goes down.
I finally received a call this afternoon from the technical support guy for the company who says this is normal and expected but he didn't know why they published the input impedance at 100,000ohm(He wasn't involved with this). He also said that future pre-amps are still going to use the same input transformers but just not tap them for the TCV and continue to use the same volume control configuration on the output so there shouldn't be any difference in sound quality.
While you are on good terms with him, try to get a schematic.
You will probably have to make some changes yourself to fix the problem.
You will probably have to make some changes yourself to fix the problem.
Yeah, I'll try...
Also the measured input impedance is already down to under 3000 ohms at 3khz.
So when an input audio transformer is rated at 10k ohm input impedance what exactly does that mean? Is it supposed to be that impedance through the whole frequency range? An average? At a specific input?
Also the measured input impedance is already down to under 3000 ohms at 3khz.
So when an input audio transformer is rated at 10k ohm input impedance what exactly does that mean? Is it supposed to be that impedance through the whole frequency range? An average? At a specific input?
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That must be from either the TVC or transformer. What is the 1kHz voltage gain at full volume?
It seems that whoever built this did not design or test it, just assemble the parts together.
This is a major failing in a basic function, and would be impossible to miss with normal practice.
It seems that whoever built this did not design or test it, just assemble the parts together.
This is a major failing in a basic function, and would be impossible to miss with normal practice.
I edited the below question in after you replied so I'll ask again in case you didn't see it.
*So when an input audio transformer is rated at 10k ohm input impedance what exactly does that mean? Is it supposed to be that impedance through the whole frequency range? An average? At a specific input?
It's also possible that the way I'm measuring isn't providing correct results although then frequency response loss with higher input impedance at the same levels as the lower input impedance measurement would then be coincidental...
I was told that they didn't have the data for the input impedance over frequency.
*So when an input audio transformer is rated at 10k ohm input impedance what exactly does that mean? Is it supposed to be that impedance through the whole frequency range? An average? At a specific input?
It's also possible that the way I'm measuring isn't providing correct results although then frequency response loss with higher input impedance at the same levels as the lower input impedance measurement would then be coincidental...
I was told that they didn't have the data for the input impedance over frequency.
I just re-ran my input impedance test while varying both the input voltage and the volume control position and the calculated input impedance was the same at around 3.5k ohm at 3KHz.
Ok, while you're set up, do a plot of impedance vs frequency as high as you can, within the limits of your equipment.
Go as low as 100Hz, if possible. Do you find that you actually need so much gain?
Go as low as 100Hz, if possible. Do you find that you actually need so much gain?
Yeah, That was my plan to see if it was linear or not. It's a bit annoying to get exact values as I don't have a variable resistor and I'm just combining various ones to create the resistance that cuts the output in half. I should be able to plot it with a few more points if it's linear.
I don't need even close to that much gain. The volume control never even touches half way. Most of my listening is done between 1/4 and 3/8 on the knob. A 1KHz test tone with 0.2V output from the pre-amp to the power amps equates to over 85dB at my listening position.
I don't need even close to that much gain. The volume control never even touches half way. Most of my listening is done between 1/4 and 3/8 on the knob. A 1KHz test tone with 0.2V output from the pre-amp to the power amps equates to over 85dB at my listening position.