Hello good people.
I've been experimenting with my Yamaha AX-400 to learn about power amp theory and did an experiment that came out with unexpected results, or at least i thought they were unexpected.
I removed C133 (LTP input bootstrap) and made new measurements after and frequency response was extended in the bass without the capacitor. Shouldn't happen the other way around?
C129 input cap was previously changed from 10uf electrolytic to 3,3uf film. Both measurements feature the 3,3uf film, just for consideration.
All other parameters, THD, noise and IMD stayed exactly the same.
thank you all.
I've been experimenting with my Yamaha AX-400 to learn about power amp theory and did an experiment that came out with unexpected results, or at least i thought they were unexpected.
I removed C133 (LTP input bootstrap) and made new measurements after and frequency response was extended in the bass without the capacitor. Shouldn't happen the other way around?
C129 input cap was previously changed from 10uf electrolytic to 3,3uf film. Both measurements feature the 3,3uf film, just for consideration.
All other parameters, THD, noise and IMD stayed exactly the same.
thank you all.
Attachments
Last edited:
The pole created by C133 and R147 is at a relatively high frequency, 16Hz, much higher than the input pole, so the purpose of the bootstrap is probably to raise the input impedance for the audible spectrum, not the subsonic frequencies.
Why did Yamaha chose it to be that way is open to speculation, perhaps having the poles too close of one another caused unwanted VLF peaking?....
Why did Yamaha chose it to be that way is open to speculation, perhaps having the poles too close of one another caused unwanted VLF peaking?....
What's the advantage of raising input impedance in this case? To better accommodate the 60kohm volume pot with less noise and/or better HF response?
What's the downside of keeping C133 out?
thank you
What's the downside of keeping C133 out?
thank you
High input impedance is always A Good Thing.
If you remove the cap, the impedance will revert to 10+39K//Zin (and the LF response will be extended)
If you remove the cap, the impedance will revert to 10+39K//Zin (and the LF response will be extended)
Perhaps also something to do with ensuring total stability seeing as the tone controls are wrapped around the power amps feedback network.
sounds like a good excuse to start the experiments with complete removal of all the "tone controls" and working with a near flat frequency response amplifier and find out what it is capable of.
Yup. Tl1 and tl2 connections to tone board removed, tl1 and tl2 shorted, r159 removed, r155 changed to 49kohm. Before freq. Response was a skewed mess.
I thought bootstrapping the input, therefore increasing input impedance would extend bass freq. response but clearly the bootstrap capacitor is too small, it seems. On higher models on the Yamaha ax range they use larger caps here, like 16uf.
I thought bootstrapping the input, therefore increasing input impedance would extend bass freq. response but clearly the bootstrap capacitor is too small, it seems. On higher models on the Yamaha ax range they use larger caps here, like 16uf.
So do you think that the bootstrap raising input impedance has something to do with yamaha using a passive preamplifier with a 60kohm volume pot on this model?
Would the 60kohm pot become a loading problem without the bootstrap cap?
Would the 60kohm pot become a loading problem without the bootstrap cap?
Have not looked at the circuit but cannot see any loading issues when a large value pot precedes a low input impedance stage. The log characteristic of the pot will be skewed to a degree but this is just an operational inconvenience. Perhaps they used such a large value pot + bootstrapping in an attempt to keep the overall input impedance high. To what end is another question.
Unless you really like listening to pipe organ music, a roll-off in frequency response <30Hz (which is what the stock amp does, -0.5db @ 30 Hz) is a GOOD thing!! This will protect your speakers from annoying and damaging super-low frequency signals. Rod Elliott even has a special circuit to do just this!:
Project 99 - Subsonic Filter
Project 99 - Subsonic Filter
where is that taking us to?
Attach your pic so that we all get to see it.
How to attach images to your posts.
Last edited:
I'm sorry.
I thought hot linking images from google photos would work as i can see them perfectly... weird. I can see them perfectly embedded in my posts on my phone also...
I'm not really wanting to remove the cap or something like that, just trying to understand it's usage and implication on noise floor, dynamics/loading, stability, or if only for filtering subsonic frequencies.
My guess is that it must have something to do with the 60kohm passive volume.
I attached the image.
I thought hot linking images from google photos would work as i can see them perfectly... weird. I can see them perfectly embedded in my posts on my phone also...
I'm not really wanting to remove the cap or something like that, just trying to understand it's usage and implication on noise floor, dynamics/loading, stability, or if only for filtering subsonic frequencies.
My guess is that it must have something to do with the 60kohm passive volume.
I attached the image.
Attachments
Last edited:
I didn't even get to see as much as that.
I just got an arrow going around in a permanent rotation.
I just got an arrow going around in a permanent rotation.
I find linked images especially annoying when going through older threads. Many old linked pictures are gone, and with them a lot of information. Those threads in more than one case became almost useless...and this is a pity.
Best solution would be to replace that 10uF electrolytic with a film cap of the same value:
a Wima polyester cap (MKS2B051001N00JO00) will probably fit nicely.
a Wima polyester cap (MKS2B051001N00JO00) will probably fit nicely.
Completely agree. Will always attach from now on.
Thank you for the suggestion but that i'm well aware of that kind of possible improvement procedure. I already have two big nice orange philips 4.7uf polyesters in there. 2.7hz corner frequency without the bootstrap, around 10hz corner frequency with 1uf bootstrap, and 0.4hz (or something likethat) with 47uf bootstrap like the Yamaha AX-500 (this one is the 400). Nice improvement in various sound qualities comparing to the old 10uf ELcap.
Anyway, the question was just trying to understand yamaha decisions on this model. AX-500 has 47uf bootstrap and is a very similar amp, almost the same circuit and topology. The top of the range AX-900 has active preamp (buffer after vol. pot and all that) and no bootstrap.
thank you all
ps: attached the current state of the amp
Attachments
Last edited:
- Status
- Not open for further replies.