Hi,
I recently bought two car tube hybrid amps.
Since my question is mostly regarding the tubes, I chose to start a thread in this section, hope that's okay.
The amplifiers are Planet Audio HVT7100 and 7150.
They were made by ZED Audio around 1997 / 2000 I believe.
Both are two channels amplifiers, the 7150 a larger and a bit more powerful and fan cooled and slightly different in design in some sections.
Anyway, I have tested and compared both.
The 7150 is perfect for me, I love its sound. It has slight laidback highs and upper mid which is great for my personal taste as my ears are very easily fatigued by higher frequencies. I could listen for hours and hours to this amp.
The 7100 however, is more pronounced in the highs and upper mid and I find it more fatiguing there. Since I am running active in my car that does not necessarily have to be a problem as I could use it for midbass/lower mid.
But I also noticed that for instance with Fleetwood Mac - Dreams, which has pretty low bass, it seems to distort or something ? Also, I thought that with some high quality piano recording some notes were distorted. Same as with the bass, there appeared to be an extra sound which shouldn't be there.
My father thinks it's impossible for an amp to distort only at certain frequencies, but since these amplifiers not only use transistors but also tubes and we basically know nothing about tubes, I was wondering whether the tubes might need to be replaced on the 7100 ?
These amps use Sovtek 12AX7 tubes, any recommendations as to trying a different model tube ?
I am going to ask these questions to ZED Audio as well, but I am always interested in different opinions from different people.
I like my sound pretty warm with slightly laidback upper mid / highs.
Thanks for your advice !
Here are some pics:
ImageShack® - Online Photo and Video Hosting
ImageShack® - Online Photo and Video Hosting
ImageShack® - Online Photo and Video Hosting
ImageShack® - Online Photo and Video Hosting
I recently bought two car tube hybrid amps.
Since my question is mostly regarding the tubes, I chose to start a thread in this section, hope that's okay.
The amplifiers are Planet Audio HVT7100 and 7150.
They were made by ZED Audio around 1997 / 2000 I believe.
Both are two channels amplifiers, the 7150 a larger and a bit more powerful and fan cooled and slightly different in design in some sections.
Anyway, I have tested and compared both.
The 7150 is perfect for me, I love its sound. It has slight laidback highs and upper mid which is great for my personal taste as my ears are very easily fatigued by higher frequencies. I could listen for hours and hours to this amp.
The 7100 however, is more pronounced in the highs and upper mid and I find it more fatiguing there. Since I am running active in my car that does not necessarily have to be a problem as I could use it for midbass/lower mid.
But I also noticed that for instance with Fleetwood Mac - Dreams, which has pretty low bass, it seems to distort or something ? Also, I thought that with some high quality piano recording some notes were distorted. Same as with the bass, there appeared to be an extra sound which shouldn't be there.
My father thinks it's impossible for an amp to distort only at certain frequencies, but since these amplifiers not only use transistors but also tubes and we basically know nothing about tubes, I was wondering whether the tubes might need to be replaced on the 7100 ?
These amps use Sovtek 12AX7 tubes, any recommendations as to trying a different model tube ?
I am going to ask these questions to ZED Audio as well, but I am always interested in different opinions from different people.
I like my sound pretty warm with slightly laidback upper mid / highs.
Thanks for your advice !
Here are some pics:
ImageShack® - Online Photo and Video Hosting
ImageShack® - Online Photo and Video Hosting
ImageShack® - Online Photo and Video Hosting
ImageShack® - Online Photo and Video Hosting
If a valve is failing then it is likely to distort at all frequencies, but as bass notes tend to be stronger signals and stronger signals will almost always distort more then your perception could be that bass only is distorting.
I assume the valves in your system are there either as fashion accessories or as part of an FX circuit so I can't help you on valve choice as I never use such systems myself.
I assume the valves in your system are there either as fashion accessories or as part of an FX circuit so I can't help you on valve choice as I never use such systems myself.
Hmm yes this could very well be the case, and might explain why I experienced the amplifier as much more fatiguing. it might be wise to check for distortion with an oscillosope and a testtone cd. What would the distortion look like on a sine wave ?
I do not think the tubes are in there just for the looks. I don't think ZED could afford to risk his reputation. They must be in the signal path.
I do not think the tubes are in there just for the looks. I don't think ZED could afford to risk his reputation. They must be in the signal path.
'being in the signal path' and 'being there for fashion' are not mutually exclusive. Some designers plop in a valve where an op-amp would do an equally good job, because they know it will help sales by increasing the price.
Distortion on a sine wave could take several forms. You might see clipping on peaks, or a wiggle near the zero crossing, or the top and bottom of the wave being a different shape (e.g. one looks sharper, the other more rounded off). A very rough rule of thumb is that if you can just about see it on a 'scope then you will probably be able to hear it without too much difficulty.
Distortion on a sine wave could take several forms. You might see clipping on peaks, or a wiggle near the zero crossing, or the top and bottom of the wave being a different shape (e.g. one looks sharper, the other more rounded off). A very rough rule of thumb is that if you can just about see it on a 'scope then you will probably be able to hear it without too much difficulty.
I've sent a big email to ZED see what he says.
I did a little test with a testtone cd with sine waves and the oscilloscope.
Got some pretty weird results on both amps !
I thought maybe I was doing something wrong, but then I took one of my normal (transistor) car amps and the signal was perfectly clean.
http://imageshack.us/photo/my-images/823/165201293357.jpg/
Here's a very ugly drawing of mine which represents what I saw on the scope.
The top half shows the smaller 7100, the bottom half the more powerful 7150.
The signal was far from clean on both amps. Strangely they sound very clean when playing music. Anyway, the first 10 seconds (after the tubes have warmed up which takes approx. half a minute) the output on the 7150 seemed clean, therefore I drew a clean signal on the bottom left, but after that it got pretty messy same as on the other amp.
There were multiple lines at the peaks, and in between the peaks they merged into a blurry thick line instead of a thin thight line. At the peaks, in between the multiple lines, the area was filled as well. With the cd on pause, the straight clean line one would normally see was very noisy.
The fact that the amps sounded different from each other was probably caused due to the fact they were not level matched (at all) when playing music. Strangely, the supposedly less powerful 7100 is way louder at the same gain setting (gains turned all the way down to 7 volt gains) than its larger brother. So it was playing much louder which explains the fact that it sounded fatiguing to me, and more present in the highs even though basically everything was louder, the highs simply annoy me rather quickly as compared to other frequencies, which might explain why I found it to be more pronounced in the higher regions.
I did test Fleetwood Mac - Dreams on both amps, and on the smaller one there is something strange going on in the bass line. I had the larger amp play louder to approximately match the output of the smaller amp, and did not notice the same peculiar thing on the Dreams' bassline.
I put the smaller one in my car, it's driving a pair of speakers which are on a 80 Hz high pass and 180 Hz lowpass and it sounds pretty damn good. I'll await ZED's mail, see what he has to say on the matter.
What's strange is that the amps sound very clean to me. I mean they could sound good to me while being noisy / distorted (or whatever is causing the unclean sine waves) if that's what I liked and I'd be fine with that if that would be the case, but besides sounding pleasing to me they sound clean, not noisy or distorted (except for that bassline in Dreams and maybe some notes in that piano track on the 7100, the 7150 does not do that).
I did a little test with a testtone cd with sine waves and the oscilloscope.
Got some pretty weird results on both amps !
I thought maybe I was doing something wrong, but then I took one of my normal (transistor) car amps and the signal was perfectly clean.
http://imageshack.us/photo/my-images/823/165201293357.jpg/
Here's a very ugly drawing of mine which represents what I saw on the scope.
The top half shows the smaller 7100, the bottom half the more powerful 7150.
The signal was far from clean on both amps. Strangely they sound very clean when playing music. Anyway, the first 10 seconds (after the tubes have warmed up which takes approx. half a minute) the output on the 7150 seemed clean, therefore I drew a clean signal on the bottom left, but after that it got pretty messy same as on the other amp.
There were multiple lines at the peaks, and in between the peaks they merged into a blurry thick line instead of a thin thight line. At the peaks, in between the multiple lines, the area was filled as well. With the cd on pause, the straight clean line one would normally see was very noisy.
The fact that the amps sounded different from each other was probably caused due to the fact they were not level matched (at all) when playing music. Strangely, the supposedly less powerful 7100 is way louder at the same gain setting (gains turned all the way down to 7 volt gains) than its larger brother. So it was playing much louder which explains the fact that it sounded fatiguing to me, and more present in the highs even though basically everything was louder, the highs simply annoy me rather quickly as compared to other frequencies, which might explain why I found it to be more pronounced in the higher regions.
I did test Fleetwood Mac - Dreams on both amps, and on the smaller one there is something strange going on in the bass line. I had the larger amp play louder to approximately match the output of the smaller amp, and did not notice the same peculiar thing on the Dreams' bassline.
I put the smaller one in my car, it's driving a pair of speakers which are on a 80 Hz high pass and 180 Hz lowpass and it sounds pretty damn good. I'll await ZED's mail, see what he has to say on the matter.
What's strange is that the amps sound very clean to me. I mean they could sound good to me while being noisy / distorted (or whatever is causing the unclean sine waves) if that's what I liked and I'd be fine with that if that would be the case, but besides sounding pleasing to me they sound clean, not noisy or distorted (except for that bassline in Dreams and maybe some notes in that piano track on the 7100, the 7150 does not do that).
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I could not look for long because almost as soon as I arrived on your pics page I got a (spurious? hopefully) warning about viruses.
'Scope traces can show all sorts of confusing things, including hum and RF pickup. A clean trace is not necessarily good; a dirty trace is not necessarily bad. It all depends on how well you know your scope and its probes.
'Scope traces can show all sorts of confusing things, including hum and RF pickup. A clean trace is not necessarily good; a dirty trace is not necessarily bad. It all depends on how well you know your scope and its probes.
I'm sorry to hear that. Though I must look up 'spurious' in the dictionary 
But the signal was clean with an other amp with the same scope.
I must admit I am not very experienced at operating and reading scopes, though.
Let's see if I can get the pic to show up in the thread. Usually, I fail on this site.
I'll try 3 different methods.
[img=http://img823.imageshack.us/img823/2369/165201293357.th.jpg]
<a href="http://imageshack.us/photo/my-images/823/165201293357.jpg/" target="_blank"><img src="http://img823.imageshack.us/img823/2369/165201293357.th.jpg" border="0"/></a>
But the signal was clean with an other amp with the same scope.
I must admit I am not very experienced at operating and reading scopes, though.
Let's see if I can get the pic to show up in the thread. Usually, I fail on this site.
I'll try 3 different methods.
An externally hosted image should be here but it was not working when we last tested it.
[img=http://img823.imageshack.us/img823/2369/165201293357.th.jpg]
<a href="http://imageshack.us/photo/my-images/823/165201293357.jpg/" target="_blank"><img src="http://img823.imageshack.us/img823/2369/165201293357.th.jpg" border="0"/></a>
It might be noise above the audible frequency range from the switching power supply. Turning up the scope gain and looking at what's there with no input signal may help. If it is high frequency, speed up the scope sweep to where you can measure the period of one complete cycle. 1/period = frequency
Some class-D amps also have significant out of band switching noise.
Software can be used along with the sound input on a computer to act as a spectrum analyzer. That can show you distortion products, but the sampling frequency is likely too low to see switching noise. (use care not to overdrive the input)
Some class-D amps also have significant out of band switching noise.
Software can be used along with the sound input on a computer to act as a spectrum analyzer. That can show you distortion products, but the sampling frequency is likely too low to see switching noise. (use care not to overdrive the input)
"Spurious" means something which is false or misleading.
Multiple scope lines often mean hum, which is not synchronised to the signal, but could also mean low frequency oscillation. Thick traces could be noise or HF oscillation. Trouble is, just putting a probe on a circuit can change its operation.
Multiple scope lines often mean hum, which is not synchronised to the signal, but could also mean low frequency oscillation. Thick traces could be noise or HF oscillation. Trouble is, just putting a probe on a circuit can change its operation.
Thanks guys I'll let my father read this as well.
I guess if the amp sounds sparkling on very revealing high quality recordings then I guess I don't have much of a problem, except for the smaller amp messing up a little on that Dreams bassline and certain piano notes, though I'd need to double check the piano with another amp, who knows it's just reproducing what's there in the recording with great detail. I doubt it however.
And the fact that the weaker amp is more powerful at the very same gain setting than the larger amp is also quite strange. Hopefully ZED will respond to my mail and I'll post the info here.
'Trouble is, just putting a probe on a circuit can change its operation.'
Indeed. I tried another amp of mine and the test did not work, nothing showed up on the scope, so who knows the test I performed with the scope and probe is messing up these particular amplifiers' proper operation.
I should have stayed in school
I guess if the amp sounds sparkling on very revealing high quality recordings then I guess I don't have much of a problem, except for the smaller amp messing up a little on that Dreams bassline and certain piano notes, though I'd need to double check the piano with another amp, who knows it's just reproducing what's there in the recording with great detail. I doubt it however.
And the fact that the weaker amp is more powerful at the very same gain setting than the larger amp is also quite strange. Hopefully ZED will respond to my mail and I'll post the info here.
'Trouble is, just putting a probe on a circuit can change its operation.'
Indeed. I tried another amp of mine and the test did not work, nothing showed up on the scope, so who knows the test I performed with the scope and probe is messing up these particular amplifiers' proper operation.
I should have stayed in school
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I'd like to take this opportunity to thank the boys and girls on the forums for being there for each other, as the reply I got from the manufacturer of these amps wasn't very kind nor helpful.
'Dear Fred,
We do not have schematics for these amplifiers as they were built about 14 years ago.
There are no spare boards available.
You are asking me to diagnose issues from 6,000 miles away based on what you hear, this is simply not possible for me.
I designed and built these 14 years ago and any listening which I did back then I certainly cannot remember.
Regards'
'Dear Fred,
We do not have schematics for these amplifiers as they were built about 14 years ago.
There are no spare boards available.
You are asking me to diagnose issues from 6,000 miles away based on what you hear, this is simply not possible for me.
I designed and built these 14 years ago and any listening which I did back then I certainly cannot remember.
Regards'
I'm thinking that riccoryder is right, those o'scope traces look like you're getting PS noise.
Trace the circuit to see where the tubes are getting their plate voltage from. Try bypassing the plate supply electrolytic cap with a 0.1mfd film cap (with equal or higher voltage rating) in parallel with electrolytic.
Trace the circuit to see where the tubes are getting their plate voltage from. Try bypassing the plate supply electrolytic cap with a 0.1mfd film cap (with equal or higher voltage rating) in parallel with electrolytic.
Hi , if you like that vintage sound (with rolled off highs) you can try swapping your tubes with Mullard , i have tryed a few tubes on my preamp and the mullards should fit your taste , Brimar are nice too but a bit more sparkly on highs , you might want to have a look at the link there´s a description on how the several brands sound like
http://www.audiotubes.com/12ax7.htm
http://www.audiotubes.com/12ax7.htm
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change the electrolytic capacitors. the metalized polyester caps are low quality, consider changing those too. as both of these have lowered shelf life since they live in a car. polypropylene caps and low impedence/esr electrolytic caps are preferred replacement. Vishay 715P series poly and Panasonic EEU are some suggestions.
replace the 1/4w carbon film resistors located on the tube riser card as these components are not suitable (they lack temperature stability) I would recommend Vishay/Dale ccf60 series.
replace the 1/4w carbon film resistors located on the tube riser card as these components are not suitable (they lack temperature stability) I would recommend Vishay/Dale ccf60 series.
With the thermal and humidity fluctuations one can see in a car, polyester caps can definitely go bad- they have greater temperature sensitivity and far greater moisture pickup than polypropylene. You'll see drifts of value and large increases in tan delta. As well, unless they are foil caps (unlikely) the metallization can corrode, leading to much higher ESR, downward drifts in value, and eventually total failure.
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