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 Post subject: Re: MCA - Son of ZCA
PostPosted: 10 Apr 2017, 18:39 
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Did you use a 0.1uf input cap? Also if you are getting clipping on one side of the sine wave try altering the bias up or down to see if you get balanced clipping.

Glad you like the amp. What is the efficiency of your speakers?

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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 Post subject: Re: MCA - Son of ZCA
PostPosted: 13 Apr 2017, 05:19 
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Hi Mark,

yes, a 100nF PP-foil is sitting at the input.
And I tried to get symmetric clipping, but it was not really working. I was guessing that a lower bias should start clipping the negative side first, but probably I didn't went low enough..
What do you think about adjusting the working point to lower voltages? May it decrease the performance because of bigger non-linearity by going a bit towards the transistor threshold or are there more/different things to consider?

By the way - the amplification bandwidth is much broader than expected. At the moment, I don't have real quantitative data, but there is no big change (not even a factor of 2) in the voltage-amplitude at up to 5 MHz.

The speakers are selfmade 3-way transmissionlines with a sensitivity somewhere between 85 and 87 dB/W. Nothing special from the sensitivity point of view..


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 Post subject: Re: MCA - Son of ZCA
PostPosted: 13 Apr 2017, 06:00 
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LuPtBi wrote:
Hi Mark,

yes, a 100nF PP-foil is sitting at the input.
And I tried to get symmetric clipping, but it was not really working. I was guessing that a lower bias should start clipping the negative side first, but probably I didn't went low enough..
What do you think about adjusting the working point to lower voltages? May it decrease the performance because of bigger non-linearity by going a bit towards the transistor threshold or are there more/different things to consider?

By the way - the amplification bandwidth is much broader than expected. At the moment, I don't have real quantitative data, but there is no big change (not even a factor of 2) in the voltage-amplitude at up to 5 MHz.

The speakers are selfmade 3-way transmissionlines with a sensitivity somewhere between 85 and 87 dB/W. Nothing special from the sensitivity point of view..


For even reasonable volume levels you need speakers of 90db efficiency. You may need to stick closer to the MCA original to get better results.

_________________
Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
Website: retro-thermionic


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 Post subject: Re: MCA - Son of ZCA
PostPosted: 18 Apr 2017, 13:01 
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Hi Mark,

you're right, efficient speakers are obviously preferable for low power amps, but I would say, if my output AC-signal is just 4V-pk-pk (whis is the case for an input signal of 1V-pk-pk), I can potentially reduce the driving dc-voltage accordingly. I could easily run the amp with an input voltage of 10VDC without getting a lower output amplitude. My only concern is about the level of distortion which might rise if I go to a lower bias because the negative halfwave will come closer to the threshold voltage.. do you agree?


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 Post subject: Re: MCA - Son of ZCA
PostPosted: 18 Apr 2017, 14:11 
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LuPtBi wrote:
Hi Mark,

you're right, efficient speakers are obviously preferable for low power amps, but I would say, if my output AC-signal is just 4V-pk-pk (whis is the case for an input signal of 1V-pk-pk), I can potentially reduce the driving dc-voltage accordingly. I could easily run the amp with an input voltage of 10VDC without getting a lower output amplitude. My only concern is about the level of distortion which might rise if I go to a lower bias because the negative halfwave will come closer to the threshold voltage.. do you agree?


Apart from building the prototype and running a few tests on it I didn't really experiment with it. Try it on the CRO and see how it turns out.

_________________
Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
Website: retro-thermionic


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 Post subject: Re: MCA - Son of ZCA
PostPosted: 03 Nov 2017, 14:09 
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Posts: 33
Hello Mark!

Long time no see. I had much on my mind a could not continue the MCA project.

But now I am back with more questions duh >)

Is it possible to modify the MCA and or ZCA to play only high frequencies? Yes you guessed it, i want to connect it to my tweeter directly, no crossover haha :)

There must be a way to modify the components of the amp to selectively make it a certain frequency range amp. I am maybe living in missbelieve but it has just a few components so it seems logical to me. I know its much simpler to just put a small capacitor to the tweeter but my tweeter (the JBL LE-20) sings much better without the capacitor.

btw: i like the idea of making one mca for the tweeter and one for the fullrange. I know it has been modified before to be used with 4ohm speakers. i just cant find the thread anymore. please tell me where to find it again, it must be another forum.


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 Post subject: Re: MCA - Son of ZCA
PostPosted: 03 Nov 2017, 21:20 
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You need to experiment but reduce the input capacitor to say 0.01uf. You should have a 0.1uf cap on the input. Start with 0.01uf and try bigger and smaller until you get the HFs you want.

_________________
Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
Website: retro-thermionic


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 Post subject: Re: MCA - Son of ZCA
PostPosted: 04 Nov 2017, 00:24 
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Joined: 23 Feb 2017, 02:02
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Had any one tried directly connecting the mosfet to the 28v supply instead? I see no reason why it should go after the collector resistor, Is the disipation above what the mosfet can handle? Thefore its nessecary to insert afetr 15ohms.
Is there a reason on why its done this way?

Since the authour said he decided to chose cap values by expriment, I recomend actually theriotically calculating them next time.
I think the input resistance is 1M+200ohms(mosfet has near infinate input resistance,at the 100k pot it is directly grounded by cap so total resistance 1m+200ohms), one can calculate the capacitance needed as input cap vs cut off freq. Output cap cutt of freq can also easily be calculated, all using this online tool if you are not familar with the calculation.
https://www.electronicproducts.com/RC_F ... lator.aspx

For transistor lovers, transistors can always be used as the driver, the approximate input resistance from the load would be (15ohms parallel with speaker res)(gain of 3055)(gain of driver) From practicall exprience ouputs normally have gain of approx 100 and drivers 200+.
The input pot 100k should become 500k up to 2M ohms to adjust voltage, 10uF and 1M ohms would be removed and middle of pot directly to driver, to allow more current flow into base of driver since it will require a bit of bias. The input pot should be as high as possible for higher input resistance, but still allowing sufficent bias curent to base of driver provding a good bias.
I'd say a 200K pot would be a good start, as the worst case load impedance would be approx 40k from a 4 ohm load.
The total input res would then be calculated load impedance in parallel with (the pot impedance to ground parallel with pot impeadance to 15ohms), input cap can then be recalculated.

For a 1Amp bias, there will be 1V accross the 1ohm and 15V accross the 15 ohms. CE voltage of 3055 would be vcc-16v.
The required voltage at the base of driver would be about 2.2V, (1V at emitter requals a voltage at base of 1v plus the be drops of 3055 and driver)
Thefore to be practicall and have good impedance and work, the base voltage must be zener, lm317, or lm431 regulated. Required voltage about 2.2V at base, you want to have the pot in middle for bias, this means a zener voltage of about 4.4V should be used.

A constant current source lm317 for biasing the 3055 should also be considered to improve efficentcy of class A operation.

Your only draw back from being a transistor lover would be lower input resistance and less ripple rejection, because the 10uF from middle of pot to ground has to be removed.
To fix this, the bias voltage source can be zener or lm317 regulated.
It could have been faster to draw a schematic, but its meaning less without explainnation. In theory this amp would work and I am more than confident, however always testing required. I will not gaurantee you operation of this circuit. Also I am not intrested in the circuit because its not audiophile and I can design way more complex ones(this means I will not consider building the circuit).

Positron wrote:
Hello Mark!

Long time no see. I had much on my mind a could not continue the MCA project.

But now I am back with more questions duh >)

Is it possible to modify the MCA and or ZCA to play only high frequencies? Yes you guessed it, i want to connect it to my tweeter directly, no crossover haha :)

There must be a way to modify the components of the amp to selectively make it a certain frequency range amp. I am maybe living in missbelieve but it has just a few components so it seems logical to me. I know its much simpler to just put a small capacitor to the tweeter but my tweeter (the JBL LE-20) sings much better without the capacitor.

btw: i like the idea of making one mca for the tweeter and one for the fullrange. I know it has been modified before to be used with 4ohm speakers. i just cant find the thread anymore. please tell me where to find it again, it must be another forum.

I would recomend instead of having 4700uF on ouput cap, you calculate cut of freq of tweet, using this online tool. You know the resistance of tweet and you know the freq you want .
https://www.electronicproducts.com/RC_F ... lator.aspx
You use the calcualted cap size and put this as outpu cap at 3055 collector.

Is this amp driving your tweet better? I'd say its the same as a power amp driving with cross over, because you always have cap on ouput. I'd say having a quality ocl amp with a high pass filter on input would do better sound quality


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 Post subject: Re: MCA - Son of ZCA
PostPosted: 05 Nov 2017, 16:16 
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Joined: 01 May 2014, 16:29
Posts: 33
Thank you both for suggestions. I will experiment with the caps.

Is this amp driving my tweeter better? I am just looking for sound that is true to the original, not smudged by too many unnecessary electronics with lots of capacitors,resistor, chips, trafos...

One more question. Can i connect two MCA (mono) amps to the left channel of my DAC and two MCA (mono) amps to the right by splitting the output signal of the DAC? Will this create distorion? Harmonics? Each amp will drive his own speaker.

Greetings and thanks again.


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 Post subject: Re: MCA - Son of ZCA
PostPosted: 05 Nov 2017, 18:27 
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No idea about the DAC_amp splitting.

_________________
Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
Website: retro-thermionic


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