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KT120 Oddblocks
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3089
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Author:  ozapaydin [ 25 May 2017, 09:05 ]
Post subject:  Re: KT120 Oddblocks

What does it mean? Can it go down to 10 Hz. with full power? I don't think so.

Author:  gofar99 [ 25 May 2017, 09:32 ]
Post subject:  Re: KT120 Oddblocks

Hi, That is not what -3db point means. It means -3 db (1/2 power) at below 10 HZ. Typically mine all get the -3 at 8HZ. Full power is available at 20HZ as well as 20K. These amps have powerful bass response. A goodly portion of it is due to the oversize output trannies. Most designs seem to skimp there with obvious consequences.

Good listening
Bruce

Author:  poty [ 26 May 2017, 03:47 ]
Post subject:  Re: KT120 Oddblocks

ozapaydin wrote:
Can it go down to 10 Hz. with full power?

I'm risking to look meticulous, but what do YOU mean for "full power"? There are several ways of measuring "max power" which show rather different results in numbers, so they can't be seen as term "full power". On the other hand, if we speak about frequency response - then you should give us the appropriate level of what you think is suitable for your estimations. The Bruce's answer can have other meaning than you are thinking about and does not mean you cannot achieve the same power @10Hz. His answer says only that "for the same input signal level the ouput level @10Hz is half of the output level @20Hz".

Author:  gofar99 [ 26 May 2017, 09:52 ]
Post subject:  Re: KT120 Oddblocks

Hi, That is a bit detail oriented. Poty is correct. In theory an amp (any amp) that can respond at a frequency with any reasonable amout of output can be driven hard enough to get the rated output at that frequency. The problem is that efficiency is lower, and distortion will raise. With tube amps there is a finite limit at which they can deliver the full output regardless of the drive. This mostly a limitation of the ability of the output transformer to transfer the power from the tube to the load. Full power as indicated in all my designs is the RMS watts delivered to the load (not on the tube side of the trannie) between what ever frequency range I indicated (normally 20HZ to 20KHZ) at the distortion level I gave. For most designs it is two percent total all forms of distortion plus noise wideband measured at any frequency within the specified power range. That is the way my HP analyzer computes it.

Now my turn, why the question?

Good listening
Bruce

Author:  ozapaydin [ 26 May 2017, 10:23 ]
Post subject:  Re: KT120 Oddblocks

Because, it's one of the parameters needed to design an OPT.

Author:  poty [ 27 May 2017, 03:41 ]
Post subject:  Re: KT120 Oddblocks

Not sure why do you need others amplifier parameters if you are going to design your own OPT?
You should know that there is also a NFB for some installations which was chosen with high-frequency resonance of the Edcor transformer in mind.

Author:  ozapaydin [ 28 May 2017, 03:34 ]
Post subject:  Re: KT120 Oddblocks

What makes an OPT ring? Can you explain NFB circuit of KT120 Oddblocks? What is the reason why you used 220pF cap in parallel with 1500R?

Author:  poty [ 28 May 2017, 06:34 ]
Post subject:  Re: KT120 Oddblocks

From the link above:
Quote:
The amplifier can run with either the minimal of negative feedback shown, or none. The feedback is used to ensure stability under possibly difficult loads as the amplifier has considerable high frequency gain without it. Therefore there is a possibility of exciting the 70-80kHz transformer resonance and causing ringing or oscillation. Use of alternate audio output transformers is not recommended for stability reasons.

ozapaydin wrote:
What makes an OPT ring?
Output transformer led, 1500 Ohm, 220pF, 1200 Ohm, 120 Ohm.
ozapaydin wrote:
What is the reason why you used 220pF cap in parallel with 1500R?
It defines max NFB in the given OPT resonance region.

Author:  ozapaydin [ 28 May 2017, 07:38 ]
Post subject:  Re: KT120 Oddblocks

I've already read the link, Poty. It's not enough to understand what causes ringing in OPT. More details please.

Author:  Suncalc [ 28 May 2017, 08:35 ]
Post subject:  Re: KT120 Oddblocks

ozapaydin wrote:
It's not enough to understand what causes ringing in OPT. More details please.
Feedback amplifiers greatly extend the bandwidth of the signal chain. At higher frequencies, circuit parameters not normally considered at normal audio frequencies become important. These include both stray impedances from layout and the overall impedance of the loop gain circuit. The output transformer contains significant elements that come into play. See figure below.
Attachment:
Output Transformer Equivelant.png
In short, the feedback function is frequency dependent. At low frequency the feedback can be negative and degenerative, just as desired. However, due to the complex impedance at higher frequencies, the feedback can become positive and hence regenerative. All output transformers have a natural resonance above the audio band. If the overall loop gain is regenerative at this frequency then the circuit becomes unstable and will oscillate in the vicinity of this resonance. Normally the feedback circuit must take this into account. Changing any one part of the loop gain circuit without understanding the circuit parameters across frequency risks introducing instability.

The theory can be better understood reading the following paper:

H. S. Black, “Stabilized feed-back amplifiers”, Elect. Eng., pp. 114–120, Jan. 1934.

The paper clearly explains how the loop gain function affects overall stability across frequency.

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