Amplifyer Setup Guide

Amplifier Setup Guide

Today all refined FM RF Archival setups are based around the vhs-rf-amp-ada4857 designed by Rene Wolf and is commonly called "ADA Amp" or just "ADA4857" amplifier in the community.

This amplifier unlike off-shelf boards from AliExpress based around 3-5v, is instead using internal 12v to 36v power from your VCR or VTR videotape decks thus allowing for direct in-unit installation.

You can order them via the KoFi Store which can be faster and supports the work, or via PCBWay if you need to make 5-10 units and can wait awhile.

You will also want to get a 0805 Resistor Assortment Book.

Tip

1% / 5% / 10% / 20% - Tolerances of parts will use different codes, always have a 3/4 digit calculator website or app handy to double-check if your confused by values in a picture.

Note

The Input impedance resistors are 2 for each path with each path using 2 resistors of the same value.

Soldering Advice & Tools

As you will be dealing with an amplifier possibly mounted inside a VCR, soldering tweezers are a cheep but invaluable tool for removing and populating resistors without any damage.

• 0805 1% Resistor Parts Book
• YIHUA 938D Portable Tweezers

1. Pre-tin the pads with a light amount of fresh solder.
2. Apply a large amount of tacky style flux on each set of pads.
3. Ensure power is off and or amplifier is disconnected while soldering parts.
4. Clean off with 99.9% IPA once finished, you can be optionally conformal coat the pads once you are 100% happy with the setup of the amplifier.

High Impedance Amplifier Input

Lick your signal, do not eat it whole.

Is the simple concept when using amplifiers on a signal, to take enough of the input electrical signal to not disrupt (due to current draw) the head amplifier/tracking ICs but enough to get the most amount of information possible from said signal.

The benefits to a dedicated amplifier on a per-deck adjusted basis, is quite simple in practice it allows for ease of use with any ADC solution that may be used to capture the signals, without worrying about adjusting or modifying gain levels on the ADC's driver amplifier.

Setting up the AD4857 amplifier

There are two parts to this process:

• finding your input termination
• finding your gain level

This is done via 4 0805 resistors.

Setting Input Resistors

Use the same value on the input path set of pads.

R11 / R12 - Input Path 1

R21 / R22 - Input Path 2

Setting Gain Resistors

R14 / R13 - Gain Setting 1

R24 / R23 - Gain Setting 2

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The below table lists Rf and Rin values, the mapping is as follows:

• Path A

  • Rf = R14
  • Rin = R13

• Path B

  • Rf = R24
  • Rin = R23

Rf (Ohms)	Rin (Ohms)	Gain	Est. upper -3dB bandwidth (MHz)	Est. lower -3dB bandwidth (Hz)
560	1000	1.6	500	22
560	560	2.0	350	22
560	270	3.0	180	22
560	180	4.1	120	22
560	150	4.7	95	22
560	120	5.6	75	28
560	100	6.6	60	33
560	82	7.8	55	41
560	68	9.2	45	50
560	56	11.0	35	60
560	47	12.9	30	72
560	39	15.4	25	87

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The upper bandwidth estimations are based on an extrapolation for the couple values from the data sheet. They are only rough guidelines and don't take into account anything but the amp IC.

So no tap point quality, PCB design, connector losses, cabling limits, etc.

The lower bandwidth estimations are based on the high pass filter of the Rin with the 47uF DC blocking capacitor.

For lower gain settings, the lower bandwidth is further limited by the 150 Ohms output load and the 47 uF DC blocking capacitor.

Voltage Levels

Once you have your input stage setup, you need to choose the gain for each signal path.

To do that, observe the video and audio RF signal levels from your tap points when playing different VHS tapes.

Different tapes will have different signal levels (test newer and older tapes, NTSC and PAL, etc).

Consulting the service manual for your player may also give you a good idea about the expected signal levels.

Based on this you now need to choose the values for R13 + R14 (path A) and R23 + R24 (path B).

Again every VCR or VRT is slightly different, but just for reference, the video levels could be somewhere around 100mVpp to 300mVpp.

HiFi RF signal levels are usually higher than video levels, so maybe 500mVpp ~ 1000mVpp.

You should also know what your target capture device needs as a good input range.

CX Cards seems to be fine with about 1.5Vpp (peak-to-peak).

So lets say you observe 0.5Vpp (500mVpp) of signal level and your device is a CX based capture card, so a target of 1.5Vpp is the goal but 0.2vpp difference is fine in most cases.

This means we need 6x gain, here's why: from 0.5V to 1.5V is a 3x gain, but you also need to have 2x for the transmission line, 2 x 3 gives a gain of 6.

In doubt select the next lower gain to get more headroom on the ADC input.

Note

An configuration aimed at the CX Cards (8-bit) should work fine for the DdD (10-bit) or MISRC (12-bit) for example if the output impedance and internal amplifiers are set to low or passthough level.

Oscilloscope Adjustments

This is pretty much identical in usage (just via dials or mouse clicks for outer interfaces)

• Great YouTube Video for analogue & digital scopes

With a digital store oscilloscope (DSO), you can easily measure and highly fine tune your amplifier based off the Peak-to-peak voltage (Vpp) levels information.

Using the OWON DSO

Firstly install your amplifier in a manner of easy access and modification, install rough values, and play an SP tape.

1. Mode --> Oscilloscope Mode
2. Measure Range --> Measure --> ON
3. Add Del --> You can then enable PK-PK (peak to peak) auto measurements and others.

To manually measure peak to peak of a fixed signal

3. Measure Range --> Cursor --> F2/F3 to select A/B and then use the UP / DOWN arrow keys to move the points.
4. The top (A) and bottom (B) points of the waveform should box in the signal
5. The AB readout is the combined "Peak to Peak" of the signal

10k-TrimPot-Adjusting-OWON-Compressed.mp4

Using the Rigol DHO800 series

1. Channel 1 --> Connect BNC (directly to test point)
2. Set Coupling to AC mode
3. Measurement Button --> Vpp
4. Set controls to 10.00ns via Vertical Dile / 500mV/ via Horizontal Dile

This should provide a clear view of an FM VHS video signal, if not simply adjust until you see a stable waveform populating the screen.

Finding A Decks Output Impedance

Rough numbers

1. Find your service manual.
2. Take the value of the resistor in the path directly after the head amplifier IC or IC's for video RF and or HiFi RF.
3. Then 10x or 20x that value is the rough guidance i.g if its 1500 or 1.5kohm you want to use 15kohm or 30kohm.

15kohm for video & 20kohm for hifi is for example common on Panasonic decks.

Measure Device Output Impedance

You will need the following tools:

• Oscilloscope

• Multimeter

• Potentiometer/Trim Pot (Virtually any 10 kOhm)

• 1x Test Point Tap or Direct Tap (either raw cable end or SMA/BNC connection works)

• 1x BNC Probe cable (hook or crocodile/alligator clip) to connect to the middle (wiper) and any of the outer pins of the potentiometer.

• 1x BNC T-connector with 1 x BNC male, 2 x BNC female configuration.

• 1x Test tape of your media format.

Setup

Connect the T-Connector to the Oscilloscope, on one end connect the RF line from your device, on the other connect your probe with a potentiometer hooked to it.

Probe cable with the potentiometer to one end and your decks RF signal to the outer.

HowToMeasureVCROutputImpedance_new.mp4

1. Start your tape, Ideally EBU or SMPTE colour bar test tape with 1khz sinewave tone HiFi audio.

2. Set the channel in the scope to AC mode, 50 - 200 mV division, and ~2.5 us time-division. (Trigger does not matter, just set to Auto or to 0 V)

3. If possible, add/enable the measurement of the peak to peak voltage of the input.

4. Turn on the VCR and play any tape. Notice that the amplitude can vary by its own. Keep that in mind when observing the amplitude while turning the potentiometer during the next step.

5. Turn the potentiometer slowly to one direction to reduce its resistance. At some point, the amplitude of the waveform on the scope will decrease alongside the Vpp.

6. Stop and now turn the potentiometer the other direction until the amplitude is back to its normal value. You want to find that position on the potentiometer, exactly when the amplitude starts to decrease just a little tiny bit.

Note

The measured peak to peak voltage can also be used at this step, as the change in amplitude is also very clearly visible.

5. Disconnect the potentiometer from the setup without turning it. During that, you can double-check that the amplitude shown on the scope does not change.

6. Measure the resistance of the potentiometer.

7. Multiply that by 10x or 20x and round to the nearest 0805 resistor value and solder that onto the path for your input signal.

Blind/Manual Fine-Tuning

Note

This segment is for if you do not have any sort of ossiliscope with measurement functions.

• Look at the service manual (if you have it), and look for resistors around the signal path you tapped normally directly after the tracking and pre-amplification IC.

Choose a value that is around 10x higher than the values you observe (e.g. 1.5k is around the signal path, go for a 15k Ohms resistor), but to be safe don't go below 1k Ohms.

If you don't have the service manual you can just start with 1k to 20k Ohms (higher the safer).

• Start with 10kOhm on the inputs and slowly work your way upto 40kOhm

• Stop at the point ware the waveform no longer changes size in an audio DAW such as ocen audio or adobe audition the waveform should be centred but not filling the entire visualised space and or have flat top or bottom parts.

Your SNR value should also have a levelling off point visually speaking in ld-analyse, however if your heads are weak you will hit a ceiling SNR wise which may be lower than the potential of your media.

It's best to have a good commercial or freshly recorded SP tape to figure out if there is a ceiling you're hitting.

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