Archive for Analog Audio

A view inside Hartke A25 Bass Combo

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Hello,

I got a used Hartke A25 Bass Combo recently and it was in good condition but the pots needed cleaning so i decided to show you what is inside this little combo.

Hartke A25 Bass Combo

Once i took out the amp i noticed the power stage is actually using discrete transistors which is really really nice. The codes are not so well visible but i think it uses 2sd2058 and 2SB1366.

The opamp used is the JRC 2068 and  a JRC 13700 is used for limiter (can be replaced with LM13700)

Hartke A25 Inside view

Hartke A25 Inside view

Hartke A25 Inside view

Hartke A25 Inside view

Hartke A25 Inside view

Hartke A25 Inside view

Hartke A25 Inside view

I also liked that the transformer is well shielded. After I cleaned the potentiometers I tested the unit and sounded great. I took some measurements to see the effect of the equalizer, range and center frequency.

I measured the output of the speaker from 50cm away and the speaker about 50cm from ground.

First i compared CD input with instrument input. Note that CD input is not affected by any of the controls, not even the master volume.

 

Hartke A25 Input compare

Red trace is the CD input and the other trace is the instrument input

Next i tested the equalizer controls.

Hartke A25 bass range

The above is the bass control. We can see it is centered around 100Hz and indeed it has 30dB control range. Blue is +15dB setting, Red is -15dB setting.

Hartke A25 mid range

 

Midrange seems to be center around 700-800Hz and again the control range is about 30dB. Blue is +15dB setting, Red is -15dB setting.

Hartke A25 Treble range

 

Treble center frequency looks to be 2kHz. The range is close to 30dB again. Blue is +15dB setting, Red is -15dB setting.

I also measure the direct output of the amp to see the frequency range of the preamp. It is quite linear as you can see below.

Hartke A25 main out

 

I can say it is well built and it has a nice tone. It doesn’t go that low but if placed on the floor it can be quite satisfying. In the future i will look more at the speaker and the enclosure as i think it can be improved a bit.

Thank you for visiting.

Marshall JCM2000 DSL401 Mod

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Many times guitarists remove power tubes in parallel PP tube amps to get more distortion, to drive the remaining tubes harder. Here is how you can do that with Marshall JCM2000 DSL401 amplifier.

Sony TA-F270 Inside

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Hello,

I had a Sony TA-F270 over for repairs and i thought i should how you how it looks inside.

Service manual is available online if you search on Google.

Sony TA-F270 inside

Sony TA-F270 inside

Sony TA-F270 inside

Sony TA-F270 inside

Sony TA-F270 inside

A pair of power transistors were busted so i replaced them.

Sony TA-F270 inside

Here is a short video on how to do it.

Thank you for visiting

Celestion G12T-100 Guitar Speaker Driver

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Hello,

I recently got a Marshall JCM2000 DSL401 in the shop for repairs. This amp is equipped with a 12 inch Celestion speaker labeled GOLD BACK. On a side label you can find the G12T-100 markings tho.

Checking to see if the problem wasn’t with the speaker itself i measured it and got the T/S parameters and impedance curve. Subscribers will find that zma and frd files are available.

Parameters (pdf): celestion g12t-100

Celestion G12T-100 Marshall Gold Back

Celestion G12T-100 Marshall Gold Back

 

Thanks for visiting

 

Guitarists, prepare for a live session

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Hello,

This post is for beginner guitarists.

Ever happened to you that you unplug the cable from the guitar? It must have! Either you stepped on the cable or you moved around too much. Here is a cheap and quick way how to take care of this problem.

Leave a loop of 10-20 cm between the clip and the jack. Adjust this length until you are comfortable and its not bothering you. When you are finished unplug the jack first then remove the clip. Keep cables in their own space and its best to hang them in long loops. Don’t wind them around your hand.

Do this every time. Don’t get lazy and leave the cable in just because you will play again tomorrow!

Thanks for visiting.

Guitar Cable Trick

Microphone Preamplifier Part 1: Restoration

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Some years ago, i got my hands on an old and obscure mono mixing console. It was labeled just as PM5200. It wasnt of much use to me so i tore it down and salvaged parts for my own projects.

The other day i was looking through my things and found the mic preamp modules from that console and i thought this would be a great weekend project. I remember i used one of the modules for a guitar recording rig. Battery powered and with a transformer input it worked very well with Shure SM57.

The circuit uses only solid state discrete components and uses capacitor coupling. It allows for 3 gain settings and a nice line in/mic feature.

Original Mic Preamp Module

Original Mic Pre PCB

I do like the shield. 1mm thick steel.

You can see the original module with the busted gain setting. First i had to do is to reverse engineer and draw a schematic. It wasn’t very difficult, old PCB single sided, resistors all 500mW clear marked. You can see the schematic below:

PM5200 MIC PRE Original Schematic

 

With +/-15V the circuit draws about 10mA so its running pretty hot.

T101 along with D101, D102, R109, R110 for a constant current source. T102 and T103 form a long tail pair. You can attack it with balanced/unbalanced signal. You can use an input for more feedback or if you use the inputs together you can get a nice line in input with about 10x gain. T104 and T105 forms the main voltage amplification stage.

Next i took down all the components. Nothing was worth keeping.

MIC PRE PCB

MIC PRE PCB2

 

I changed the NPN transistors to 2SC2240 from Toshiba. I really like these transistors in audio applications. PNP transistors i used BC559C, low noise and i have alot of these.

Also i replaced R119 with a multiturn pot of 5k in value. I hope i can get rid of the last coupling capacitor. We will see. One thing that surprised me was the lack of any power rail decoupling on board. I added 2 100n MKT caps.

Modified Mic PReamp

Modified Mic PReamp PCB

 

After that i recorded frequency response which you can see below. It is pretty linear with just 1dB dropping at high frequency.

PM5200 Mic pre frequency response

 

In next part i will explore some improvements and just put it in a box, add controls and make it ready to be used.

Thank you for visiting.

 

Balanced Line Attenuator バランスラインアッテネータ

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Hello,

In a home recording environment equipment often doesn’t come in a large variety because of either limited budget or limited space… or both. I am presenting here an balanced attenuator which comes in between fixed gain preamp and recording device/soundcard.

The purpose of this device is to adjust the level and monitor it to prevent saturation of the next stage in the recording chain. It uses a L-pad followed by a balanced buffer stage. The attenuation steps are 0dB, -3dB, -6dB, -9dB and -12dB.

The meter section uses a microcontroller with a 10 bit ADC. It monitors both polarities of the signal and detects the peak within a frame of 1000 samples. ADCs samples the signal every 12us.

Balanced line attenuator schematics バランスラインアッテネータ

 

In the schematics above J7 will be used later on future revisions to indicate symmetry in the balanced signal.

J3 will connect the attenuation selector.

There are certain modifications i did the initial schematic tho. R7, R8 must be of greater value. At least 100k. With 100k you will get a -0.8dB signal at 0 dB setting.

The circuit is pretty straightforward, you can use any quad opamp chip for U3 as long at it operates from a 9V single supply. I do recommend a FET input opamp if R7 and R8 >= 100k as they tend to have lower noise than bipolar with high input impedance.

U2 i used a LM324 as it allows operation at 5V single supply. U2:C is used to bias the ADC inputs at 2.5V.

I will explain how to rectify the signal with a PIC in a future project but you will find the hex file for this one at the end of this article.

Here are the PCB drawings:

バランスラインアッテネータ Balanced Line Attenuator

pcb top layer

バランスラインアッテネータ Balanced Line Attenuator

pcb bottom layer

Balanced Line Attenuator バランスラインアッテネータ

pcb top silk

 

Warning: NONE of the PCB images are mirrored!

Here is how the PCBs turned out:

Balanced Line Attenuator バランスラインアッテネータ

Balanced Line Attenuator バランスラインアッテネータ

Balanced Line Attenuator バランスラインアッテネータ

Balanced Line Attenuator バランスラインアッテネータ

I used a rotary switch to select the attenuation levels. For 0dB you can just omit R15. Use POT2 to calibrate 0dB on your meter. I usually set it to 0dBV.

Balanced Line Attenuator バランスラインアッテネータ

Balanced Line Attenuator バランスラインアッテネータ

 

After building the first unit i made some measurements. I used 5% tolerance resistors so i wanted to see if the attenuation levels are correct. Here are the results:

Balanced Line Attenuator バランスラインアッテネータ

 

-6dB and -12dB settings are about 1dB off and it seems -3dB setting is also a bit off. For -12dB i soldered a 47k resistor in parallel with the 6k8 one and for -6dB i soldered one 100k resistor in parallel with the 22k. For -3dB i soldered a 470k resistor in parallel with the 47k one. Results were much better:

Balanced Line Attenuator バランスラインアッテネータ

 

I will be making kits available for this device with PCBs and programmed microcontroller as well as fully built units. Please subscribe to receive more details about this offer in the next newsletter.

Below you can find the hex file:

PrjRyu Balanced Attenuator

Thank you for visiting

 

Monitoring Amplifier モニターアンプ P4: VU Meter Driver

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Hello, こんにちは,

In this part I will continue with the VU-meter driver for needle instruments. The circuit is very simple just a half-wave rectifier with adjustable gain and a comparator for clip indicator.

POT3 should be chosen to match your meter’s sensitivity but i think 100k will do for most. Components are not critical so it should be fairly easy to build.

Whats important to note is that the voltage at the inverting input of the LM339 comparator will set the clipping threshold, so you can use a voltage divider there to set it to what you want.

You will need to adjust POT3 to your preferred 0dB point, I usually use the dBV scale.

vu meter schematic

Monitoring Amplifier Vu meter circuit アンプVUメーター回路を監視する

Monitoring Amplifier Vu meter circuit アンプVUメーター回路を監視する

 

Monitoring Amplifier Vu meter circuit アンプVUメーター回路を監視する

 

Slowly but steady the whole unit is coming along. In next part i will do the source selection and front end.

Parts List:

Quantity: References Value
3 R1, R3, R5 10k
1 R2 1k
1 R4 680-1k
1 C1 10u
1 C2 3.3u
1 U1 TL072
1 U2 LMV339MT
1 D1 1N4148
1 D2 LED
1 J1 CONN-SIL3
1 POT1 10k
1 POT2 4.7k
1 POT3 100k-470k

Thank you for visiting

TRS to XLR Interconnect Cable

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Hello,

If you have been around audio enough you know that accessories, while many times ignored, can become really costly. Interconnect cables are easy to built and you can apply simple techniques to make them much better than the more expensive commercial alternatives.

In this article i will make a TRS – XLR balanced line interconnect. The connection follows the rules described below:

TRS      TO        XLR

Tip                   Hot

Ring                Cold

Sleeve            Gnd

In choosing the cable i recommend to look for braided shield and cloth layer. These features will help get a reliable cable.

TRS - XLR Interconnect Audio Cable  TRS - XLRインターコネクトオーディオケーブル

TRS - XLR Interconnect Audio Cable  TRS - XLRインターコネクトオーディオケーブル

The picture above depicts how i like to prepare the cable for the TRS plug. You can see the braided shield which will connect to the sleeve of the TRS plug.

TRS - XLR Interconnect Audio Cable  TRS - XLRインターコネクトオーディオケーブル

TRS - XLR Interconnect Audio Cable  TRS - XLRインターコネクトオーディオケーブル

TRS - XLR Interconnect Audio Cable  TRS - XLRインターコネクトオーディオケーブル

TRS - XLR Interconnect Audio Cable  TRS - XLRインターコネクトオーディオケーブル

In the picture above you can see my favorite way of securing the plug by using the cloth to tie it around the sleeve terminal.

TRS - XLR Interconnect Audio Cable  TRS - XLRインターコネクトオーディオケーブル

TRS - XLR Interconnect Audio Cable  TRS - XLRインターコネクトオーディオケーブル

I always use heat tube to isolate the shield connection of the cable when using XLR plugs. The XLR terminals are even in height so the shield will have a few mm of bare wire. I want to avoid it to touch the plug chassis on its own so i isolate it.

To solder the cable to the plug it is a good practice to hold the plug vertically like in the picture above.

TRS - XLR Interconnect Audio Cable  TRS - XLRインターコネクトオーディオケーブル

And the finished cable.

 

Thanks for visiting.

Monitoring Amplifier モニターアンプ P3: Speaker Coupling Delay

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Hello, こんにちは,

私の日本語があまり上手くありませんが許して下さい.

In this article i will present a simple delay circuit that will be used to couple the speakers to the amplifier after a certain settling time was allowed. The circuit also allows for to be controlled by an external 5V logic signal. This can be used to decouple the speakers in case a fault is detected.

これは、アンプがオンされたときにスピーカーを保護するためのDELAY回路であります。それが徐々にコンデンサを充電することによって機能します。コンデンサ電圧が一定値に達したときには、リレーを切り替えります。

The circuit schematic is presented in figure 1 and as you can see it uses just discrete components. It is a linear voltage ramp generator that commands a power transistor. The current charging capacitor C1 and the capacitor’s value are the parameters that set the ramp’s slope.

POT1は、時間遅延を調整します。

Speaker delay circuit schematic

Figure 1

In figure 1 Q3 forms a constant current source adjustable via POT1. R1, R2, D1, D2 set a voltage on the base of Q3 of about 5.4V and this means about 6V voltage drop over R6 and POT1 series connection. Assuming Ic3 = Ie3=Icharge,

Icharge = 6V/(R6+POT1)

Lets set POT1 at 90kohms for ease of calculation. This gives R6+POT1 = 100k.

Icharge = 60uA

Since Q3 is in saturation mode we can assume a voltage drop over C-E of about 0.5V so the voltage over the capacitor Vc1= 5.5V. The time for the capacitor to be charged to 5.5V is defined by the below equation:

T= (C1*Vc1)/Icharge = 0.91 second

上記の式は充電時間コンデンサを計算します。

Q2 buffers the voltage across C1 capacitor. It also provides a small delay until Vc1 reaches around 0.6V to bias Q2’s B-E junction. Q1 acts as a switch and when turned on via a 5V signal it absorbs most of the current from Q3 and capacitor will not be charged.

Q4 has the role to drive the relay. It is a small power transistor and it’s enabled via POT2. This variable transistor has the role to set the on/off steps based on the ramp voltage. If too low the relay will be on very fast and stay on if too high the relay will never activate.

Speaker delay circuit schematic

Figure 2

In figure 2 the time step is 200ms and we can see the ramp is about 1s long, very close to what we calculated. The blue trace is the Fault signal. When a 5V pulse is present the capacitor C1 is discharged very fast (pink trace) and speakers are decoupled (green trace). When the fault signal goes to logic low or ground the ramp generator shortly starts the process and enables the relay after about 1 second.

Speaker delay circuit schematic

Figure 3

 

Speaker delay circuit schematic

Figure 4

Figure 3 shows how the relay is activated faster if the POT2 is set too low in value and figure 4 shows a correct setting. The yellow trace represents power switched on.

Below you can see the circuit in the right side of the board.

Speaker delay circuit schematic スピーカ遅延回路図

 

Parts list does not contain the connectors in the schematic because the circuit most likely will be used as a part of something bigger:

Part Quantity
BC549 2
BC559 1
BD139 1
1N4001 3
R 12K 5% 0.25W 3
R 10K 5% 0.25W 2
C 10u 25V 1
POT 250K 1
POT 10K 1
RELAY DPDT 1

Thank you for visiting,

ご覧いただきありがとうございます。